This study attempted to explore the association, if any, between academic and social motivations including goal, value, belief and expectation factors and their influences on academic achievement in science among the junior-high school students. Two questionnaires, namely “Social Expectation Questionnaire” (SEQ) and “Internal Learning Motivation Questionnaire” (ILMQ), were developed in the study to assess students’ motivations. By factor analysis, SEQ and ILMQ received a satisfactory validity and reliability. In the main study, 277 students in the third year of junior high school (the ninth grade) were invited to take the SEQ and ILMQ, and the correlation and regression analyses were applied to probe the associations between the motivational factors and effects on school science performance. The result showed that subscales of social and academic motivation were correlated with one another and contributed differently to the science learning result assessed by the national standardized examination. By regression analysis, seven motivational factors that predicted the result of the standardized examine were abstracted.