Measurements were made on the mean and fluctuating velocities in a turbulent boundary layer without pressure gradient, but with streamwise vortices introduced artificially by a series of vanetype vortex generators. Effect of the streamwise vortices is observed explicitly from spanwise variation of the profiles of mean and fluctuating velocities. In particular, the shape factor of the mean velocity profile (the ratio between the displacement and momentum thicknesses) exhibits values lower than that of the undisturbed flow in the majority of the spanwise sections, suggesting the tendency toward delaying boundary layer separation. The mean streamwise vorticity is mostly concentrated in the vicinity of the vortex core, and this is accounted for as produced by the anisotropy of the normal Reynolds stresses. In the adjoining spanwise section there exists also a region of counter-rotating streamwise vorticity, which is weaker in strength but inhibits the diffusion of the primary streamwise vorticity. Both the anisotropy of normal Reynolds stresses and the generation of counter streamwise vorticity appear to contribute to the mechanism by which the streamwise vorticity could persist for a long distance.