Deformation resulting from residual stresses is a key problem in selective laser melting (SLM). The different melting pool conditions between the deformation and normal specimens are compared based on data collected by the melt pool monitoring system. Furthermore, the forming mechanism and deformation process of the specimens is investigated. Results from the experiments suggest that deformation of the specimens occurs in the initial stage of the process. The melt pool area decreases and the intensity of the melt pool increases. The experiments indicate that it is possible to detect deformation of the specimens by monitoring the melt pool behavior.