Diplomatic relations between the Tran Dynasty and the Yuan Dynasty stretched over a hundred of years, but the early 40-year period - a period associated with the three resistance war against the Mongol invaders - was the most severe and complex. Unlike the Song Dynasty, the Yuan not only had stronger military power, but its ambition was also much larger than the former. Thus, early in the diplomatic relationship with the Yuan, the Tran Dynasty, on one hand, absorbed the diplomatic lessons from the previous dynasties, but, on the other hand, couldn't carry out a tough diplomatic policy like that of the Early Le and Ly Dynasties, but smoother and more flexible. Through diplomatic papers, we can see the consistency in the foreign policy of the Tran, which was the uncompromising attitude of sustaining the national independence and sovereignty, the combined hard and soft policy which skillfully combined diplomatic struggle with the struggle on the political front to dissipate the Yuan's ambitions of invading Vietnam. That uncompromising attitude was expressed in all diplomatic papers and letters. Through them, it can be recognized that the Tran Dynasty's foreign policy was extremely flexible. The style in diplomatic letters was profound and watertight, sharp but humble with strong argument, which is worth a pattern for the following generations.