This article describes the effect of reticle flexure caused by clamping on pattern positioning accuracy. Reticle flexure causes a pattern shift resulting from stretching or compression of the reticle surface. We focused on the clamping mechanism of the pattern writing system and the measuring machine which held the reticle at four points. By repeating the clamping of the reticle, it was recognized that the surface height distribution deviates. As a result, pattern shift deviation occurs. To improve the repeatability of clamping, a new three-point clamping mechanism was developed for electron-beam writing systems, and also a three-point adapter was employed for the measuring machine. The Values parameters related to pattern positioning accuracy, x and y scaling, and orthogonality were calculated for several reticles (5 in. sq., 0.09 in. thick). By clamping the reticle at three points, the deviations of these three values were reduced to less than 53% that of four-point clamping. Moreover, surface shift caused by three-point clamping was precisely compensated by adding a compensation circuit to the electron-beam writing system.