This paper describes some experiments with the thermal deformations of workpieces, caused by the heat generated in surface grinding, and their influences upon the geometrical accuracies of finished workpieces. The thermal deformations and the temperature distributions of workpieces have been measured directly by using strain gauge type pickups and thermocouples. The straightness of finished workpieces have been measured and compared with the.deformations during grinding. The influences of some parameters on the thermal deformations are investigated. From this study, the following results are obtained. The temperature differences are produced between the top and bottom surfaces of workpieces during grinding operations. Consequently, the workpieces are deformed into convex shape, in addition to the simple expansion in the direction of thickness. As the result, the profiles of finished work-pieces become concave. The cooling effects of grinding fluid are remarkable, but insufficient. The suppressing effects of the electromagnetic chucks and the normal grinding forces on the.thermal deformations are comparatively small.