Based on finite volume method, three-dimensional models are used to evaluate the effect of flow restriction device (plug) on outlet temperature and efficiency of LS-2 parabolic trough collector. In order to study the effect of plug, various positions of plug with different diameters are used as case-studies. In other hand, solar heat flux distribution on the outer wall of the receiver tube is calculated by Monte Carlo ray tracing method (MCRT). The MCRT method is applied and coupled with three-dimensional numerical methods, computational fluid dynamics (CFD), in order to study the effect of plug diameters and their positions on outlet temperature and efficiency of collector. The result of MCRT method shows nonuniform heat flux hits on outer wall of receiver tube. So, after simulation of absorber tube of LS-2 PTC, the nonuniform heat flux is applied in the computational code. In order to validate the numerical methods, the working fluid and physical simulated model and operating conditions are considered as Syltherm-800 and LS-2 parabolic trough collector which had been tested via Dudley et al. at Sandia National Research Laboratory (SNRL). After the validation of numerical method, several case-studies with variable plug diameter and plug positions are simulated. Other working fluids are also tested for modeling of mentioned case-studies too. Results show that if the amount of nondimensional displacement from center becomes +0.5, then outlet temperature will be gentler. It is independent of plug diameter and working fluid. Finally, the efficiency of each of cases is evaluated. Consider that, the evacuated receiver tube is utilized in simulation of parabolic trough concentrator (PTC) and therefore, the convective losses have been negligible.