English is regarded as an International Language. Most Thai students have to learn English as a Foreign Language (EFL). Significantly, learning languages, not only English, is related to a foreign culture's behavioral norms, and cultural values that usually determine a person's self-identity. Especially, students in English major may undergo self-identity changes. This quantitative study investigated types of self-identity changes occurring in undergraduate students majoring in English, at Naresuan University (Eng. NU) and Thammasat University (Eng. TU), also students majoring in English language and literature at Thammasat University (Eng. Lit). In addition, this study investigated the factors which had influenced these self-identity changes. The instrument of the research was a questionnaire. ANOVA was carried out to test difference and analyze the correlation between six types of changes and six independent variables (t-test). (Contains 3 tables and 3 figures.)