<p>Producing a flat part layer with the stable melting mode is crucial for the laser-based powder bed fusion of metals (PBF-LB/M) to manufacture parts with high dimensional accuracy. This study aims to clarify the effects of preheating by varying the hatch distance on the melting mode during PBF-LB/M under a support-free condition. Ti-6Al-4V powder was irradiated by laser scanning 100 paths at various hatch distances. During the melting process, a two-color pyrometer was used to measure the temperature distribution on the powder bed. At smaller hatch distances, the melting mode transitioned from a keyhole mode to a conduction mode. However, at larger hatch distances, the melting mode remained unstable until laser irradiation was completed. Temperature distribution revealed that laser irradiation not only melts the powder but also promotes powder preheating on the nearby paths owing to the heating by thermal conduction between the powder particles. The preheated and presintered powder improves the apparent thermal conductivity. Consequently, the melting mode transitions from the keyhole mode to the conduction mode owing to suppression of the vaporization of the powder particles. Based on the findings, we developed two simple thermal energy balance models, which predict the melted mass per unit area and the criterion for the transition of the melting mode depending on the hatch distance. The results using the simple thermal energy balance models roughly matched the experimental results.</p>