This study examined the characteristics of male high school English learners' motivation and the relations between their motivation and achievement in English learning. A total of 334 Grade 10 students in a boys' high school participated in this study by completing a questionnaire survey on second language (L2) learning motivation and taking an English achievement test. As a result of explanatory factor analysis, five factors were found as underlying constructs for male high school students' English learning motivation: The Ought-to L2 Self, The Ideal L2 Self, The Feared L2 Self, Instrumentality, and Intrinsic Motivation. In terms of the structural relationships between the learners' motivation and achievement in English learning, the ideal L2 self and intrinsic motivation exerted a direct impact on their motivated L2 behavior, and then English achievement mediated by the motivated L2 behavior. The other factors had an indirect influence on motivated L2 learning behavior, mediated by the ideal L2 self and intrinsic motivation. Based on the findings, we suggested motivational activities to realize the creation of the ideal L2 self among male high school English learners.