A series of waterborne polyurethane/oil fly ash composites were prepared with different content of OFA through in situ polymerization. A homogeneous OFA distribution was found for all composite coatings. The mechanical strength, adhesive strength, and wet resistance all improved using proper OFA content (above 1wt%). The coated specimens were immersed in artificial salt water for one month. The adhesive strength changed for all of the coatings in this condition. However, the coating adhesive strength was almost unaffected, when a 2wt% OFA content was used.