The effect of boundary geometry on the properties of a rarefied vapor flow emerging from a narrow and infinitely long source was studied. The boundary consisted of walls, along both sides of the source which were parallel to the bulk velocity direction and of a collimator which plane was perpendicular to this direction. Two‐dimensional numerical solutions for the flow field were obtained based on the Bhatnagar–Gross–Krook model. It was found that the rarefied flow is affected mainly by the size of the collimator aperture and by the walls separation. The streamwise length of the walls and the location of the collimator have a minor influence on the far field density. ©1996 American Institute of Physics.