The course of processes of hydraulic classification and also processes of separation (enrichment) depends on the differences in the velocity of falling of grains. The falling velocity is conditioned, among others, by dimensions, density and grain shape. In the case of separation of materials in which all grains have the same density and almost similar dimensions the grain shape may be the factor determining the falling velocity. In the bulk processes where we deal with many grains of differentiated shapes, this factor can have a significant effect on the process efficiency. The paper presents the results of model investigations on the influence of grain shapes on the separation process in the jig. A narrow grain class (15 to 30 mm) of the valley gravel was used for the tests which was a mixture of grains of close-to-similar density (about 2.55 to 2.68 g/cm3) and the mass from 3 to 5.5 g. The selected grains were characterised by a clear differentiation of shape. The grain shape was described conventionally by the "shape indicator" and was determined as the relation of grain mass to the perpendicular projection of its area on the plane, at its most stabile position assumed by the grain after its free projection on the surface. It was an attempt to combine the grain shape with its mass as a factor affecting the velocity of displacement of grains. The values of the "shape indicator" were from 8 for flat grains and psheriacal, through slightly elongated and needle-shaped, up to 27 for spherical grains. It was assumed that for such a material the grain mass and shape would be the factors which determine the separation of grains in gravitational processes. The falling velocities of grains of almost similar masses were determined experimentally, depending on their shape, and the separation was performed in a device which imitated a jig. The investigations confirmed the effect of the grain shape on the course and results of the gravitational separation. In case of the narrow grain class the effect of the grain mass on the results of separation was less significant. The separation time and grain shape constitute an important parameter. Spherical grains were subject to be arranged in order faster than flat grains. Their participation in the upper layer of the sediment of the jig decreased and, depending on the separation time, was from a few to 0%, at the simultaneous increase of their participation in the lower layer to about 66%.