Aim: ;The aim of this study was to evaluate the degree of conversion (DC) of composite resin at different depths of simulated immature root canals using light-transmitting plastic post (LTPP) and three different light sources. Methodology: ;Composite resin was packed into 60 black plastic cylinders 12 mm in length with 4 mm internal diameters to simulate immature root canals. LTPPs were inserted into half of the simulated canals and the other half acted as controls. Both the simulated canals with LTPPs and the controls were divided into three groups of 10, and each group was cured using either a quartz-tungsten-halogen (QTH), light-emitting diode (LED), or plasma arc (PAC) curing unit. Specimens were sectioned in three horizontally 24 h after curing to represent cervical, middle, and apical levels. DC for each section of composite resin was measured using a Fourier transform infrared spectrophotometer, and data were analyzed using three-way anova and Tukey tests. Results: ;At the cervical level, no significant differences were found between specimens cured using different light sources or between specimens with and without LTPPs (P > 0.05). However, DC was significantly higher in specimens with LTPPs than in those without LTPPs at both the middle and apical levels (P < 0.05). The mean DC of all specimens with LTPPs was significantly higher than that of specimens without LTPPs (P < 0.05). PAC unit showed lower DC than QTH and LED units at both the middle and apical levels; however, the differences were not statistically significant (P > 0.05). Conclusions: ;The results of this study suggest that the use of a LTPP increased the DC of composite resin at the middle and apical levels of simulated immature root canals, but that DC was independent of type of light source.