This study examines the pattern and extent of actual vs. perceived math achievement gaps (test scores vs. self-ratings) among Koreans, Korean-American immigrants and Korean-American natives. The study links TIMSS 2007 8th grade math data and ECLS-K 2007 8th grade math data together for matched sample (Korean ethnicity) comparisons. In terms of actual achievement as measured by test scores, Korean students perform significantly better than Korean-American students who in turn perform better than other American students. In contrast, in terms of perceived achievement as measured by self-ratings, Korean students have significantly lower self-concept than both Korean-American and other American students. The findings imply that Korean students are akin to "a small frog in a big pond", whereas Korean-American students are more like "a big frog in a small pond." This study gives new insights into multi-faceted, transnational achievement gaps between student groups with common ethnic background but different frames of reference and academic standards.