Continuously overlapping a number of tracks resulted-in a significant influence in laser nitriding of a Ti-6Al-4V alloy. Not only was the microstructure in the overlapped areas found to be different from that in the non-overlapped areas, but also the microstructures developed by successive tracks could influence each other, especially for the first few laser tracks. Moreover, the melt-pool depth and profile changed with the sequence number of the overlapped tracks. This resulted in a non-uniform surface layer and therefore non-uniform surface properties. The correlations between the processing parameters and melt-pool depth and hardness were determined, which leads to a method or principle for the design and control of the processing whenever track overlapping is required. A second important consideration is the effect of specimen thickness on the resultant microstructure. A CO2 continuous spinning laser beam was used in this work.