Parenting knowledge is regarded as an important variable influencing parenting style. This study investigated the parenting knowledge of mothers with infants and analyzed how this knowledge influenced their parenting style. The mothers’ parenting efficacy was included in the analysis as a mediator between parenting knowledge and parenting style. Three hundred and seventy-five mothers with infants completed questionnaires regarding parenting knowledge, parenting efficacy, and parenting style. The results showed that the mothers reported different scores in subcategories of parenting knowledge, in which the highest scores were in knowledge about rearing behavior and the lowest were in the developmental process. Second, there were differences in parenting knowledge scores according to the age, employment status, and educational level of subjects. Third, subjects with the more accurate parenting knowledge reported more positive parenting efficacy and parenting style. Lastly, parenting efficacy completely mediated between parenting knowledge and parenting style.