Thin plate-like 4045 aluminum alloy sample is fabricated by laser melting deposition (LMD). Microstructure and microhardness of the laser melting deposition 4045 aluminum alloy before and after heat treatment are analyzed. Results indicate that a hypoeutectic microstructure comprising about 42.7% (volume fraction) of directionally solidified Al dendrites with the primary dendrite arm spacing of approximately 22.3 μm and the secondary dendrite arm spacing of approximately 7.5 μm is developed during LMD process. The transition from columnar Al dendrites to equiaxed Al dendrites takes place at the top of the as-deposited sample. Coral-like Si develops at the bottom of the depositing layer and changes to flake -like Si at the top of the depositing layer due to the decrease of the cooling rate. Flake-like Si should be sufficiently remelted out during the subsequent layer deposition to obtain well modified coral-like Si. Coarse equiaxed Si develops at the overheated zone just below fusion line due to the spheroidizing and coarsening of coral-like Si, and the microhardness of the overheated zone is lower than that of the inner zone. Coral-like Si can be totally spheroidized after 540 ℃ isothermal heat treatment for 20 min, and Si coarsens and grows up leading to the decrease of the microhardness as the holding time extends.