It is generally accepted that cognitive ability predicts academic achievement, and that parental involvement and expectations form part of the constellation of factors that predict their children's academic achievement, particularly for families within the Chinese-heritage Cultures. Although a number of interactions between these parental factors have been proposed, the mediation effects of parental expectations on their children's cognitive ability in predicting academic achievement are yet to be established. Data from 780 students from one primary school in Hong Kong and their parents were used to generate structural equation models to test the hypothesis that parental affective factors, as indicated by parental home and school involvement, parental beliefs of their children's ability and parental expectations of their children's academic scores, mediate the effects of student IQ score in predicting school achievement in English, Chinese and Mathematics. The results support the hypothesis that parents help their children to actualize their cognitive ability by directly communicating their academic expectations to their children.