In the present study we examined how 3 to 5 year-old children collaborate in a block play. We observed and videotaped 18 same-age and same-sex pairs' play sessions: each session lasted 20 minutes. The collaboration in those play sessions was analyzed from the viewpoints of themes and episodes. An episode was defined as children's individual act when building something with blocks, exploring surrounding buildings, telling a story, and making believes. A theme was defined as the framework showing what children construct or what play they do as a whole. 18 cases were classified, according to existence and kind of each pair's theme, into the following four types: "no-common-theme", "similar-theme", "common-theme through blocks"(where an operation with blocks was effective in arranging the common theme), and "common-theme through ideas"(where an idea preceded operations with blocks in arranging the common theme). There were more tried common themes in older children than younger. Episodes were classified according to not only the commonness of pair's episodes but also each other's way of the comprehension of episodes. Age trend in the episode level was unclear. When patterns of collaboration were classified according to the states of collaboration during the entire 20 minute-session, several patterns were found in children of the same age. The states of collaboration were closely related to the theme types. Each theme type included different contents of episodes and different arranging and comprehending processes of episodes. That is, in no-commontheme group, almost all episodes were constructions and the rate of uncomprehended episodes was high. In similar-theme group, the rate of the common episodes was as low as that of the no-common-theme group but the rate of requesting of each other's comprehension showed an increase. In this type, operations with blocks were particularly effective in arranging the common episodes. In common-theme through block group, the rate of the common episodes was so high that common plus comprehended episodes occupied more than 2/3 of the whole episodes. Operations with blocks mainly enabled the arrangement of the common episodes. Children often appealed to each other explicitely when requesting to comprehend episodes. In common-theme through ideas group, the rate of the common episodes together with the rate of requested but uncomprehended episodes were high (the highest of all 4 theme types). The common episodes were mainly arranged through the exchange of utterrances. Finally, our results were compared with those of Parten et al.(1943), Smith(1978), etc. Parten's linear model as states of collaboration in children's play correlating with their age was found not to be proper in this study. Even 3-year olds who shared the common themes, were able to develop the common episodes frequently, while 5-year olds who did not share the common themes comprehended few episodes. From the results of this study parallel play was considered to comprise a phase of the flow in the group play rather than a lack of immaturity.