In the early stage, the fixed exchange rate policy was established in Taiwan, with focus on the exchange of NT Dollar to US dollar. After undergoing the changes of flexible exchange rate system, the regulation of exchange rate gradually renovates. On January 30, 1991, the exchange rate system changed to a managed floating system that allows the exchange rate to be more liberal. The spot USD trading price is no longer restricted by the upper or lower limit among banks, and the negotiation of trading price is completely free. As the exchange for NTD to USD becomes more liberal, the issue of the factors behind the price fluctuation on NTD to USD has become an interesting subject to study. This paper investigates Taiwan¡¦s foreign exchange market in order to discover the factors that cause the price volatility, whether it is private information or macroeconomic news announcement of public information. This study examines the exchange rate occurred every 15 minutes during January 5, 1992 to November 27, 2001. Given the result that the increase of macroeconomic news announcement does not increase the volatility, the volatility in Taiwan¡¦s foreign exchange market is mainly caused by private information, not public information. Although the return variance is comparatively higher than the return variance in other normal time period during the macroeconomic news announcement, the highest return variance before the trade close does not occur at the time of public news announcement. It represents that the occurrence of volatility is not affected by the macroeconomic news announcement. If foreign exchange volatility is not affected by macroeconomic news announcement of public information, then private information might be the major factor affecting the price volatility. The findings are as follows: 1. The volatility in trading period is much higher than the volatility in non-trading period, demonstrating the existence of ¡§exchange message effectiveness¡¨. Meanwhile, it also states that public information is not the only information existing in the market. Even at the most efficient market, the informative pricing has reflected all the public information. The macroeconomic news announcement of public information would not affect the price volatility, the asset pricing volatility is affected by the private information. 2. Trading time become longer which makes the informed trader not necessary to trade in a hurry, diverging the volatility of transaction. 3. The volatility at closing period increases because of the occurrence of private information. It may downgrade to public information during non-trading period. People holding valuable private information would trade before the market is close. Concluded from above, it can be discovered that the private information has played an important role incurring the large volatility in Taiwan¡¦s foreign exchange market.