To measure precise of polarization need a strong polarized absolute calibrator on the sky to accurately set polarization angle and the cross-polarization leakage. As the most intense polarized source in the microwave sky at angular scales of few arcminutes, the Crab nebula will be used for this purpose. so, we processed the Stokes I, Q, and U maps in order to compute the polarization angle and linear polarization fraction of the Crab nebula at 90 GHz by IRAM 30m telescope.