One of the main features of globalization is the increasing mobility of population. As an immigrant society, Hong Kong has witnessed waves of Mainland Chinese arrivals and assimilation into her mainstream, particularly around the change of sovereignty period. School-aged children constitute a substantial fraction of the new population. Given their potential impact on Hong Kong's present and future, how these immigrant youngsters perform in school has always been a concern to the public as well as the government. This study compared Hong Kong 8th-graders' mathematics performance in the four rounds of the Trends in International Mathematics and Science Study by their immigrant status. The results showed that the first-generation immigrant students' performance had obvious retrogression compared to native students in the past years. Among the natives, non-local-born students with only their father born in Hong Kong had the most similar performance as the first-generation immigrants. The causes for the differences are explored from both internal and external perspectives. (Contains 4 figures, 2 tables, and 5 notes.)