Aim: The aim of this study was to evaluate the effect of two types of custom tray materials, (Poly-methyl methacrylate resin, and thermoplastic splint material) on the dimensional accuracy of stone master casts. Materials and methods: Three clinical situations were suggested single crown ( preparation on maxillary right second premolar), three unit maxillary fixed partial denture (missing maxillary right first molar ) and a five unit maxillary fixed partial denture ( missing maxillary right first molar and right first premolar). Five samples were made for each clinical case using each custom tray and poured into type IV extra hard dental stone. Measurements were determined for each specimen at predetermined locations (interabutment, mesiodistal, buccolingual and occlusogingival dimensions) and recorded using stereomicroscope (10x magnification). Repeated measures ANOVA test was used to compare between main measurements of models obtained from impression trays and the master model. Tukey's post-hoc test was used for pair-wise comparisons. There was no statistically significant difference between the two types of tray materials tested. Conclusion: No statistical significant difference between the two types of trays. It was concluded that the accuracy of master casts obtained from impressions made with the tested thermoplastic vacuum formed polyethylene custom tray at the three clinical conditions under investigation, were comparable with those made with chemically cured poly methyl methacrylate custom tray.