The purpose of the present study was to examine the validity of modeling science achievement in terms of 3 social psychological variables (school connectedness, science attitude, and active learning) and 2 self-perception variables (self-confidence and science value). Two models were tested: full mediation and partial mediation. In the full-mediation model, effects of the 3 social psychological variables upon science achievement were hypothesized to be completely mediated through science value and self-confidence. In the partial-mediation model, however, those 3 variables were hypothesized to affect achievement directly as well as indirectly through the mediating roles of science value and self-confidence. Data were obtained from Grade 8 Saudi students (N = 4,099) who participated in TIMSS 2007. The relationships among constructs were examined with the use of structural equation modeling software Mplus7. Results indicated that both models performed adequately in terms of fit indices, but the partial-mediation model was retained due to its superiority over the full-mediation model in representing the sample covariance matrix as tested through chi-square difference test. The mediating role of self-confidence in the relationships of science attitude and active learning to achievement was substantiated, but the mediating role of science value was not supported.