The objective of this study is to characterize the effect of in situ heat treatment on polylactic acid (PLA) and plasticized PLA during fused deposition modelling (FDM) with the motivation to improve their mechanical, thermal and physical properties. The in situ heat treatment was formed during the FDM by adjusting the bed temperature to 70°C, 90°C and 120°C. The performance of 3D, the printed samples, was compared with the compression moulded samples treated at the same temperature using a vacuum oven. PLA was plasticized with poly (ethylene glycol) (PEG) at different compositions of 0, 5 and 10 wt% of PEG. The properties of PLA, plasticized PLA and FDM-printed sample were analysed using Instron, differential scanning calorimeter (DSC) and X-ray diffraction (XRD). It was found that the addition of PEG into PLA decreased the tensile strength, elongation at break and tensile modulus of the materials. However, after heat treatment in the vacuum oven and FDM, the properties were generally higher at 90°C and this was believed to be primarily due to the effect of crystallization. This can be proved by formation of double melting peak, correspond to melt-crystallization mechanism, observed in DSC. The formation of different crystal was supported by XRD analysis where the amorphous peak had transformed into sharp peak at 16.9° and 19.3°, which indicates an improved crystallinity. Comparison between the compression moulded sample and FDM demonstrated that the in situ heat treatment in FDM had the most significant impact on tensile modulus.