Hyperbranched polycarbosilane with allyl end groups was synthesized via hydrosilylation of methyl-diallyldilane, and characterized by Fourier transform infrared spectroscopy, H-1, C-13, Si-29 nuclear magnetic resonance, and size exclusion chromatography/multiangle laser light scattering. The degree of branching and average number of branches of the resulted polymer determined by Si-29 NMR spectroscopy is 0.58 and 0.42, and the exponent alpha in Mark-Houwink equation is 0.33 based on the relationship between viscosity and molecular weight. UV curing behaviors of the hyperbranched polycarbosilane were investigated using differential scanning photocalorimeter, and the effects of diluent's concentration, light intensity, reaction atmosphere, and temperature on the curing behaviors and kinetics were studied in detail. It was found that curing reaction can be accomplished rapidly under UV irradiation within 40 s both in air and in nitrogen atmosphere if acrylic reactive diluent was employed. The result suggests that it is an effective way to increase the curing reactivity by incorporating acrylic reactive diluents with high UV sensitivity into the polycarbosilane system. (C) 2007 Wiley Periodicals, Inc.