To modify water absorption properties of bamboo fiber (BF) and high density polyethylene (HDPE) composites, heat treatment of BFs was performed prior to compounding them with HDPE to form the composites. The moisture sorption property of the composites was measured and their diffusion coefficients (D-m) were evaluated using a one-dimensional diffusion model. Moisture diffusion coefficient values of all composites were in the range of 0.115×10(-8) to 1.267×10(-8) cm(2)/s. The values of D-m decreased with increasing BF heat-treatment temperature, and increased with increasing BF loading level. The D-m value of 40 wt% bamboo fiber/HDPE composites with BFs treated with 100 degrees C was the greatest (i.e., 1.267×10(-8)cm(2)/s). Morphology analysis showed increased fiber-matrix interfacial bonding damage due to fiber swelling and shrinking from water uptaking and drying. The mechanism of water absorption of the composite, indicated a general Fickian diffusion process.