This study investigated the relationship between self-efficacy and imposter phenomenon and their effects on EFL learners' writing ability. Participants were 94 male and female university students. The aim was to find out how self-efficacy and imposter phenomenon would be related, how these personal factors would relate to writing ability, and whether gender and proficiency level would moderate between the personal factors and writing. The instruments were Writing Self-Efficacy Scale, Harvey Imposter Phenomenon Questionnaire, Oxford Quick Placement Test, and a written essay on a specified topic. The findings revealed self-efficacy and impostorism were positively related, but only writing self-efficacy could predict students writing ability. While proficiency level affected writing ability, gender differences had no effects.