The present study explored the different levels of motivation, the relationship between the three components of the L2 Motivational Self System and language proficiency as well as the reasons behind the students' motivation in learning at two private English schools in Phnom Penh, Cambodia. One-hundred and twenty students with different levels of English, ages ranging from 9 to 21 years old, were divided into low and high achiever groups. They participated in the quantitative data collection phase. Data were collected through an English proficiency test and a questionnaire. The results revealed a significant difference regarding Ought-to L2 Self and the L2 Learning Experience between the two groups. A significant correlation between the Ideal L2 Self and language proficiency was found among high achievers, whereas a significant correlation between the Ought-to L2 Self and the L2 Learning Experience was the case among low achievers. For the qualitative study, twenty out of one hundred and twenty participants (10 low achievers and 10 high achievers) were interviewed for reasons behind their learning motivation in learning English. Different reasons were discovered: personal aspiration, the need for communication and environment.