Background: Thermoplastic immobilization devices are used to position the patient on the table in order to correctly reposition the patient during treatment courses. Objective: The Purpose of this work is investigating the degradation of surface dose and the dose distribution in the build-up region for photon beams associated with immobilization devices using Gafchromic films. Materials and Methods: After heating, these masks are stretched and fitted over the considered location of body before treatment simulation for insuring the reproducibility of patient position during treatment fractions. In this research, dosimetry was carried out using Gafchromic EBT3 film and three kinds of thermoplastic masks (Orfit with thickness 2.2mm, holes diameter 2.5mm, Orfit with thickness 2mm, holes diameter 1mm, and Klarity mask, thickness 2mm, holes diameter 3mm). Measurements were made with and without the mask materials on the surface of the Perspex phantom for 6 and 15 MV X-ray beams of a LINAC machine. Results: The results showed that surface dose increases 2.1 to 6.7 times and 2 to 3.9 times than the surface dose in the open field for 15 MV and 6 MV photons, respectively. According to the obtained results from the Analyses of Variances (ANOVA) test , it is defined that there is a significant difference in surface dose among three kind of thermoplastic masks (χ2=49.78 and df=3 and P<0.0001). The surface dose in Klarity has a  significant difference in comparison of other masks according to PostHoc exams and there is no significant difference among two other masks (P>0.05). Conclusion: According to the results, Klarity mask is more acceptable immobilization device when compared with other masks in the test.