Influence of post annealing on the microstructure of hot dip aluminum coating on 45 steel was investigated. The wear properties of HDA steel after diffusion annealing at 1000℃ was also examined. Results show that HDA coating consisted of brittle phase Fe₂Al₅ as the diffusion annealing temperature was below 900℃, however which tranformed to ductile phase FeAl and Fe₃Al at 1000℃and the coating exhibited a structural integrity and good bond with the substrate. The wear rate of the coating annealed at 1000^oC dropped significantly as the environment temperature increased from room temperature to 200℃ and 400℃, reaching an extremely low level. At room temperature, the wear rate rapidly increased with loads. At 200℃ the wear rate hardly changed with loads; the average wear rate was 4.2×10^-6 mg/mm. At 400℃ under the loads of 50 –200 N, the wear rate was lower than that at 200℃ under the corresponding loads. HDA steel presented excellent wear resistance at 200–400℃, owing to a 1–2 μm thick tribo-layer formed on the worn surface, mainly composed of Al₂O₃, Fe₂O₃, and a bit of oxides of W and Mo. In this case the coating suffered mainly from oxidation and slight wear. However, at 400℃ as the load increased to 250 N, the tribo-layer peeled off because of which became unstable , thereby the coating spalled off, and the substate was plastically extruded.