Introduction Fused deposition modeling (FDM) is an additive manufacturing (AM) process that provides physical objects commonly used for modeling, prototyping and production application by using the layer by layer deposition of a feedstock filament material such as thermoplastic polymers and composite based polymer (1-5). The advantage of FDM includes the ability to produce complex geometrical parts, low cost, easy operation and etc. The major drawback of this process is poor mechanical property due to the porous structure of final parts. In this research work the physico-rheological behavior of PLA polymer during FDM process was studied experimentally and with numerical model. The influence of process parameters such as feed rate, nozzle temperature and platform temperature on the neck growth between the filaments was investigated. Effect of nozzle temperature: In the experimental study, as in PLA sintering, in two-filament at isothermal conditions, the neck growth velocity increased with increasing filament temperature. Effect of cooling rate: In addition, the effect of the cooling rate on the polymer sintering was also observed. The higher the cooling rate occur the smaller connection between the polymers. Effect of platform temperature: Clearly, rising platform temperature will increase the polymer temperature due to the heat transfer to a warmer surface. Effect of feed rate: The influence of feed rate is more important than both platform temperature and nozzle temperature. As the feed rate increases, printing time reduces and so better layer formation between the filaments. Modelling The coalescence of two filaments is verified first by Bellehumeur model (6) in isothermal condition. It was seen that this model predicts well the sintering phenomenon. Then for FDM process as the temperature evolution is not isothermal. The model was modified by definition of an equivalent viscosity and equivalent surface tension. The constants of model then were determined. References 1) Helena N Chia Benjamin M Wu, “Recent advances in 3D printing of biomaterials,” J. Biol. Eng., vol. 9, no. 4, pp. 1–14, 2015. 2) S. H. Masood, Advances in Fused Deposition Modeling. Reading, Massachusetts: Elsevier, 2014. 3) J. Cantrell et al., “Experimental Characterization of the Mechanical Properties of 3D Printed ABS and Polycarbonate Parts,” 2017. 4) M. Nikzad, S. H. Masood, I. Sbarski, and A. Groth, “A Study of Melt Flow Analysis of an ABS-Iron Composite in Fused Deposition Modelling Process,” Tsinghua Sci. Technol., vol. 14, no. S1, pp. 29 37, Jun. 2009. 5) J. Zhang, X. Zhou, W. Wang, and Y. He, “Materials & Design Numerical investigation of the in fl uence of process conditions on the temperature variation in fused deposition modeling,” Mater. Des., vol. 130, no. March, pp. 59–68, 2017. 6) O. Pokluda, C. T. Bellehumeur, and J. Vlachopoulos, “A modification of Frenkel’s model for sintering,” AIChE J, vol. 43, pp. 3253–3256, 1997.