The purpose of this research was to study and compare the reliability of Allen’s method (ΩW) and McDonald’s method (ωMD) using a NEO Five – Factor Inventory Scale and three different extractions with three different sample sizes. The sample was freshmen of Rajamangala University of Technology in Bangkok area. The results of the research were as the reliability of multidimensional scale from different extractions within the different sample sizes when using ΩW and ωMD were different with statistical significance at the level of .05. The estimation of reliability of multidimensional scale was most appropriate when using 3000 pseudo populations as the representative of large sample. The extraction of generalized least squares or principal factors with ΩW should be used with the sample sizes of 100 and 200 while the extraction of principal factors with ΩW should be used with the sample size of 500.