A major challenge for multinational companies is to motivate employees with different individual cultural characteristics and national cultures to share knowledge. Although comparative studies across different countries have been conducted, little is known about the effects of individual cultural differences in this context. The purpose of this study is to investigate the effects of individual and national cultures in knowledge sharing. The individual cultural characteristics of power distance, individualism/collectivism, and uncertainty avoidance are incorporated into the model as antecedents of knowledge-sharing motivations (organizational rewards, image, and reciprocal benefits). National cultural differences are examined by subjects conducted in the U.S. and China. Results show that power distance is significantly related to reciprocal benefits for the U.S. but not for China. Individualism/collectivism is related to organizational rewards and image for the U.S. but not for China, while individualism/collectivism is significantly related to reciprocal benefits for China but not for the U.S. Uncertainty avoidance is significantly related to reciprocal benefits for the U.S. but not for China. This study provides knowledge-sharing practices and managements for multinational companies attempting to motivate U.S. and Chinese employees to share knowledge.