In order to study the coalescence mechanisms of thermoplastic polymer powders, a 2D mathematical model has been established based on Frenkel, Eshelby and Pokluda's model. Sintering experiments have been carried out by using two polyamide fibers that can be considered as infinite cylinders with its length much larger than the diameter. 2D mathematical model has been validated through comparison with results of sintering experiments as well as Constrained Natural Element Method (C-NEM) coalescence simulation. This consistence shows that the proposed coalescence model and experimental results can provide a reference for the numerical simulation of sintering process.