The author developed a wideband precise I/Q modulator using GaAs pHEMT technology. In this technology, pHEMT has 0.22µm metallurgical gate length and f_t=51GHz at V_ds=5V. With the careful design of the wideband phase shifter, this IQ modulator achieved a large wideband frequency range of 250MHz to 8GHz and good EVM performance after calibration. For overall frequency range, low distortion performance is obtained, where third order intermodulation is less than -42dBc. Also the ACPR at 2.2GHz for W-CDMA application is less than -74dBc.