A low cross-polarization reflectarray antenna using substrate integrated waveguide (SIW) slot antenna as element is presented. The proposed slot antenna element consists of two SIW layers, and a longitudinal slot is etched on the upper layer for radiation, while the SIW section on the bottom layer has variable length for phase controlling. A coupling slot is etched in the metal interface of the two layers to realize signal interconnection. In the whole frequency band, the phase of the proposed element can be linearly adjusted over the range of 360°, with small phase error of less than 20° under different oblique incident angles (<30°). Moreover, a very low cross-polarization level of less than - 60 dB is realized for the element. Using the newly proposed SIW element, a novel reflectarray antenna is designed and fabricated. The simulated and measured results show that the proposed antenna exhibits a high maximum gain of 24 dBi at 30.4 GHz with the aperture efficiency of 48% and a low cross-polarization level of -40 dB.