Dicumyl peroxide (DCP) was used to improve the melt flow index of polypropylene and polyethylene composite by thermal degradation in order to enhance the melt flow index of bamboo fiber reinforced polypropylene and polyethylene composite. The micro-crosslinking induced by DCP greatly enhanced the mechanical property and reduced the moulding shrinkage of bamboo fiber reinforced polypropylene and polyethylene composite. The results show that DCP could greatly promote the degradation of polyolefin and the melt flow index of polypropylene and polyethylene composite corelate linearly with DCP concentration. The addition of bamboo fiber could narrow the variation of melt flow index induced by DCP concentration. The composite with bamboo fiber concentration of 45% could obtain the best mechanical property at the optimum DCP concentration of 0.3%.