The purpose of the present study was to explore the trajectories of negative emotionality of infants aged 1-3 and affective parenting behaviors of their mother, and then to examine their bidirectional effects between the two variables. The longitudinal effects of negative emotionality and maternal affective parenting behaviors in infancy on externalizing problem behaviors when they were 4 year olds at preschools were also examined using latent growth modeling. In order for this purpose, data from 1,325 children whose mothers participated in 2nd to 5th phases of the Panel Study on Korean Children were analyzed. It was revealed that the level of negative emotionality was likely to increase whereas the level of mother’s affective parenting behaviors seemed to decrease from aged 1-3. Also, higher initial negative emotionality at age 1 predicted the rate of decreases in mother’s affective parenting, whereas higher affective parenting of mothers tended to decrease their child s negative emotionality during infancy. The initial level (intercept) of maternal affective parenting behaviors had a negative influence, and the rate of change (slope) of negative emotionality had a positive effect on externalizing problem behaviors. This study provides evidence that affective parenting is an important factor in preventing preschooler s externalizing problem behaviors.