This study examines the efficiency and informative roles of information and its effects on stock prices in Thailand when a firm announces a winning of a business contract. Business contract approval announcements usually influence stock price and are crucial to investors because they explicitly and directly increase a firm's future cash flow without distortion by the managerial process. The efficiency role proposes that public announcements will have no or relatively low effect on stock returns, while the informative role suggests that stock returns are affected by announcements. More specifically, if the efficiency role has any weight in the explanation of abnormal returns, it should be observed relatively smaller abnormal returns on winning government contracts and larger size contracts to corporate and smaller size contracts. But, if the informative roles play a significant weight in the explanation, relatively larger abnormal returns on winning government contract and larger size contracts should be observed. This study reveals that business contract announcements yield positive effects on stock returns, both on and after the event date. The findings also suggest that corporate and small size contract announcements show higher cumulative abnormal returns relative to the cumulative abnormal returns on governmental and larger size contract announcements. These results support the efficiency role of winning business contract announcement information. © EuroJournals Publishing, Inc. 2012.