Using social network analysis, we investigated the characteristics of social networks composed of positive relationships (positive network: PN) and negative relationships (negative network: NN) in classrooms of Japanese 3- and 4-year-olds. Analysis of "density" showed that PNs were denser than NNs among 4-year-olds but that this was not the case among 3-year-olds. The difference between the probability of dyads of girls forming cliques, between PNs and NNs, was larger than that for dyads of boys or mixed-sex dyads. Four-year-olds formed cliques more often in PNs than in NNs, compared to 3-year-olds. This study showed that both sex combination of dyads and age affect the quantified properties of social networks among preschoolers. (Contains 4 figures, 3 tables and 2 footnotes.)