Taiwan is the type of island economy that highly depends on international trade. Taiwan geographically closes to Japan and they are belonging to the same Asian culture so that trade frequently. According to the international trade theory, the exchange rate is an important factor influencing trade. According to Purchasing Power Parity theory, the relative price of the two countries is an important factor in determining the exchange rate. Therefore, what's the causality and time lag between exchange rate and relative price of Taiwan and Japan? The study tries to investigate this theme.This study adopts unit root test, Vector Auto-regression model and Granger causality test to analysis the exchange rate and relative price of Taiwan and Japan. The research period is from January, 1991 till December, 2011.