In this study, we examine the property difference of preliminary financial reports by regulation types using 1,425 observations provided by DART database ranged from 2006 to 2008. Preliminary financial reports are announced mandatorily or voluntarily before actual earnings are finalized at the general shareholder meeting in Korean stock market. There are two kinds of preliminary financial reports by regulation types in Korean stock market. The first type of the preliminary financial report is called ‘Timely disclosure’. Firms are required to disclose preliminary financial reports when they face the material change in the current net income compared to the prior net income. The other type of the preliminary financial report is called ‘Fair disclosure’. Firms voluntarily disclose information regarding earings forecasts for analysts and investors before they are audited. The main difference between the two types of preliminary financial reports is whether it is mandatory or not. As stated, the ‘Timely disclosure’ is mandatory while the ‘Fair disclosure’ is voluntary. Prior research suggests that such preliminary financial reports have a tendency to be optimistic compared to actual earnings. One of reasons suggested by prior research is that there would be no serious penalty for the over-estimation if firms disclose the restatement of preliminary financial reports. In addition, market participants show significant reactions to preliminary financial reports, meaning that preliminary financial reports are useful in their decision making. In other words, market participants take the information from preliminary financial reports into account when they make investment decisions. However, those studies do not take regulation types into account in examining the properties of preliminary financial reports. Moreover, their definitions of preliminary financial reports are generally mixed. Therefore, it may have additional empirical implications to examine the properties of preliminary financial reports by regulation types, such as Timely Disclosure and Fair Disclosure, especially in that the two types of disclosures show a significant difference: mandatory versus voluntary. The difference is likely to affect various aspects of financial reporting. We also investigate firm characteristics for the restatement of preliminary financial reports by using 652 restatements of preliminary financial reports. In order to test our empirical hypotheses, we include regulation-type variables in OLS regressions for the properties of preliminary financial reports. From this test, we find that preliminary financial reports for Fair Disclosure is more accurate and less biased, compared to preliminary financial reports for Timely Disclosure. In addition, we regress the bias and accuracy on firm characteristics variables, such as firm size, leverage, ROE, and market to book value. From this test, we find that the larger firm size is, the more accurate restatements are, while the accuracy of restatements decreases when firms are more leveraged, and less profitable. Finally, we find that restatements are informative. The findings of this study suggest that managers make less effort and take advantage of restatements when disclosing preliminary financial reports for mandatary Timely Disclosure,compared to for voluntary Fair Disclosure. In other words, the findings imply that the regulation type (mandatory versus voluntary) affects the properties of preliminary reports. We believe that this result provides insights not only for potential investors but also for regulators.