By using a defocused optical field with a Ronchi grating to get a quasi-sine optical field and by using a grating π phase shifting technique, the fundamental spectrum modulated by the object height distribution can theoretically be eXtended from O to 2fo (fo is the carrier frequency of the optical field) without overlapping the zero component and other higher spectra. Consequently, the measurable slope of height variation can be extended to nearly three times that of the unimproved Fourier transform profilometry method. This paper gives the theory and experiment resplts of the improved Fourier transform profilometry.