Effects of early social experience with peers and siblings on preschoolers' social competence with peers were examined, using quantification method of the first type. The mother's ratings for the questionnaire were used to estimate early social experience. Social competence was multidimensionally evaluated, and 3 composite measures (initiative competence, cooperative competence, aggression) were created. The following results were obtained: 1) Nineteen point three-31.2% of variance of each social competence measure was predicted by variables of early social experience. 2) The quality of experience with peers was more predictive of social competence in kindergarten, compared with the amount of experience with peers. 3) Early directive-directed peer experience was related to later initiative competence. 4) Early passive and/or cooperative peer experience was related to later cooperative competence. 5) The amount of early experience with siblings was related to later aggression. These results indicated that effects of early social experience were different for each of social competence measures, and that the quality of early experience was most important on the development of social competence.