Background and Context: Understanding the effects of learner characteristics and perceived learning environment on achievement in academic fields including Computer Science (CS) is of critical importance. Objective: This study aimed at testing a hypothesized model of achievement in CS in terms of the learner and the learning environment characteristics. Method: Data were collected using a questionnaire administered to a random sample of 315 eleventh and twelfth-grade advanced CS students (28% girls). Structural equation modelling (SEM) analysis was utilized to test the proposed structural model. Findings: The hypothesized structural model fits the data reasonably, yet five of the 17 assumed effects were not significant. A modified model with only significant effects fit the data well and accounted for 41% of the variance. Mathematics achievement, self-efficacy and classroom learning environment are the most influential variables on achievement in CS. Implications: The findings bear important implication for helping students by resolving obstacles that obstruct their learning and achievement.