The purpose of this study was to examine the effect of a structured, playful activity program on the development of social skills in preschool aged children (4-6 years) during break time in the schoolyard of kindergarten. The participants were 40 pupils (26 toddlers and 14 pre-toddlers, 26 boys and 14 girls) from one Greek public kindergarten. Twenty (20) of them were set, as they were divided into their school classes, as control group (CG) and the rest 20, as experimental group (EG), who received activities intervention for four weeks (2 days a week). Data on children's social skills (cooperation, interaction, independence) and behavior problems (externalizing and internalizing) were collected from their teachers before and after interventional program, by completing the Preschool and Kindergarten Behavior Scale (PKBS-2, Merrell, 2002). The results indicated that both groups exhibited significant improvement in the investigated social behaviors. Nevertheless, in the final measurement the intervention children showed significantly higher improvement in the externalizing problems than the control group. The findings highlight the need for early detection of developing behavioral problems and the positive effect of a playful activity structured program in the schoolyard during breaks on the development of preschoolers' social skills.