Non-woven hemp fiber mat has been used to reinforce unsaturated polyester to make natural fiber composites. Thermal properties of the hemp fiber mat were investigated to discover the range of heat treatment temperatures suitable for the hemp fiber mat. Loss of weight during heat treatment and absorption of moisture from the environment during storage of the hemp fiber mat were also studied. Both hand lay-up technique and compression molding were used to make hemp mat composites. Due to the low fiber fraction, no significant reinforcing effect was found in the composite made by the hand lay-up technique. The effects of heat treatment of fibers, water content in the fibers, fiber fraction, and manufacture methods on tensile properties of the resulted composites were investigated. Hemp mat composites with tensile strength and modulus comparable to those of [±45°]4 glass fiber reinforced polyester were achieved by compression molding at a molding pressure of 2MPa.