<p style='text-indent:20px;'>This study investigates information sharing in two-tier supply chai-ns considering cost reduction effort and information leakage, with either upstream competition (system SC) or downstream competition (system RC). Results show that in system SC without information leakage, the retailer shares information with one supplier when suppliers are efficient in cost reduction, shares information with neither supplier when suppliers are inefficient in cost reduction, and shares information with two suppliers when suppliers are intermediate in cost reduction efficiency. nformation leakage won't affect the information sharing decisions of the retailer. In system RC with or without information leakage, both retailers share information with the supplier when the supplier is efficient in cost reduction and neither retailer shares information with the supplier when the supplier is inefficient in cost reduction. However, the threshold of cost reduction efficiency without information leakage is always lower than that with information leakage, which demonstrates that it is less likely for retailers to share information with information leakage. What's more, the two retailers choose the same information sharing strategies without information leakage but the opposite information sharing strategies with information leakage when the cost reduction efficiency is intermediate.</p>