Recently, the position sensitive proportional counter (PSPC) has been becoming popular as a detector for X-ray stress measurement. However, little information is available in the literature regarding the effects of specimen mis-setting on the stress measurement using the PSPC.In this paper, a modeling of the Ω assembly X-ray stress measurement using a PSPC as the detector is presented enabling us to simulate the stress measurement under the various conditions including specimen mis-setting. As the results of simulation, it was found that the use of collimator yields an apparent stress (intrinsic stress), and that the absolute value of the stress increases with the increase in the width of the collimator and decreases with the increase in its effective length. It was also found that the error in stress measurement due to specimen mis-setting is expressed as a linear relation of the ratio L/R0 (L: offset of mis-setting, R0: goniometer radius), and that the coefficient in the relation is given as a function of secondary order of the ratio 2bψ/Cl (2bψ: width of collimator, Cl: effective length of collimator).