Dr, Sun Yat-sen’s decision to align with the USSR and to cooperate with the CCP in his later years has critical influences on modern Chinese history. What were the factors that led Sun to make this decision which the KMT repudiated completely soon after his death? This writer attempts to delve into historical facts as well as employ decision-making approach to tackle this question. It is hoped we may thereby better understand both the domestic and international situations which motivated Sun to take this oft misunderstood step. In the preface we first emphasize the ultimate goal of Sun’s revolutionary ideal, i.e., the national unification. Secondly we notice the current public opinion of the time which was sympathetic to the Russian revolution, and the intellectual tendency during the May Fourth movement which was more of less socialistic in its overall inclinations. Internationally we notice the disappointment of Sun with the attitude of certain majors powers, especially the USA, Japan and the Great Britain that Sun had relied heavily before. At that time it seemed the Soviet under the leadership of Lenin was the only power that was willing to offer a helping hand. Given Sun’s ultimate goal of shaking off the imperialist yoke that was imposed on China for so many decades, eliminating the evils of warlordism that had inflicted so much misery on the Chinese people, and building a unified and strong China that can claim equality in the family of nations, we may say that Sun’s policy of alignment with the USSR and cooperation with the Chinese Communist was an expedient strategy that should not be judged by later developments.