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The methodology and results of experimental study by Sachs method of the effect of length of mandrelled thickwalled steel (0.5 % C) cylinders with inner diameter of 5 mm, outer diameter of 15 and 25 mm on residual stresses are studied. It is shown that such effect can take place both due to the uneven process of mandrelling along the cylinder length, and due to zero axial residual stresses on its ends. It is found that the length of cylinders has the strongest influence on axial residual stresses. With allowances between 1.9 and 13,7 % and length reduction from 40 to 10 mm, the largest absolute value of axial residual stresses decreases from 210 to 50 MPa. It is noted that when the outer diameter of cylinders is 15 mm its length significantly influence on both hoop and radial residual stresses. With the above-mentioned reduction in cylinders length, absolute values of hoop and radial residual stress in the region adjacent to the hole, depending on the mandrelling allowance, can increase (from-135 to-205 MPa and from-45 to 55 MPa respectively with 7,1 % allowance), decrease (from-315 to-235 MPa and from-135 to-95 MPa respectively with 0.9 % allowance) and remain almost unchanged (with 3,4 % allowance). The effect of length on these stresses is weak when the outer diameter of cylinders is 25 mm.