This study intended to improve the performance of the beta-type Stirling engine, being developed by the authors for solar energy and low-grade heat sources, by means of displacer surface treatments. Three different displacers were manufactured and tested where one of them was without any surface treatment, other was zirconium coated with 0.15 mm thickness, and the other was helically knurled with 0.30 mm track depth. Because of good thermo-physical properties, helium was used as the working fluid. The heat was supplied by an LPG burner. Tests were conducted at 360 +/- 10 degrees C hot end temperature. The highest engine power was obtained with knurled displacer as 250 W at 545 rpm speed and corresponding to this power 4.38 Nm torque was obtained. This was followed by coated and smooth displacers. Power increments provided by the knurled displacer are 40 and 60% compared with the zirconium-coated and untreated displacers. Increments of knurled displacer's torque compared with that of coated and untreated displacers are 13 and 30%, respectively. Copyright (C) 2009 John Wiley & Sons, Ltd.