Some oak species show synchronous production of large seed crops within a population of trees, so-called masting. Several hypotheses have been proposed to explain the mechanism of masting, often in relation to the weather condition. In Quercus crispula, masting is considered the result of high survival of flowers to mature fruit within 1.5 month after flowering. High survival of flowers is considered to relate to high temperature in the early stage of flowering. To evaluate whether masting is the result of high temperature, we conducted a two-year study of flower and acorn production in Q. crispula with a branch-warming experiment. Using a canopy crane, we attached electric heating cables to branches in the top of mature 4 trees, and elevated branch temperature 5°C higher than control. The number of flowers maturing into acorns was counted from May to September every 3 weeks in 2009 and every 6 weeks in 2010. Branch warming slightly increased survival of flowers and acorn production. Global warming may have the possibilities to provide a considerable change in the feature of acorn production in some oak species.