Considering that achievement gaps have become a serious educational problem worldwide, the Ministry of Education in Taiwan has been addressing a series of policies to reduce achievement gaps, but the effect of these policies has not yet been carefully examined. Therefore, the present study investigated current educational policies, achievement gap phenomenon, and its changing trends in the use of national standardized tests in Taiwan and evaluated the degree to which these policies are reducing the achievement gaps among Taiwanese students. Junior high school graduates who took the high school entrance examination between 2004 and 2010 were recruited to examine the associations between their academic achievements and variables such as socioeconomic status (SES), ethnicity, and school district. Time series analyses were conducted to determine the longitudinal trends for the achievement gaps. The results revealed that achievement gaps are decreasing among different ethnicities, but they are increasing among different SESs and school districts. These findings demonstrate some positive effects of the educational policies introduced in Taiwan, but also the need for further modification and precise implementation of these policies. Suggestions for implementing educational programs and policies to reduce achievement gaps are also presented.