Self-evaluation is not innate

but developed gradually under the influence of postnatal activities.The process of learning is a gradual process of making blind trials and correcting errors

which is the theory of "trial and error"（hereinafter referred to as "trial and error"）.Development of positive self-evaluation in children relies much on continual "trial and error" in family activities.This paper introduces contents and relevant experiments of the "trial and error" theory

elaborates the concept of self-evaluation and its role played in selfconcept

analyzes internal relations between the "trial and error" in family activities and the self-evaluation of school age children in detail

and makes several suggestions to promote "trial and error" in school age children.