A novel technique to generate a microwave vector signal based on a bidirectional use of a polarization modulator (PolM) and a phase modulator (PM) in a Sagnac loop is proposed and experimentally demonstrated. The I/Q data streams of a vector signal are mixed with a microwave carrier and then applied to the PolM and the PM, respectively. A microwave vector signal is generated by combining two microwave signals with one obtained by beating the optical carrier and the +1 st order sideband of a phase-modulated signal and the other obtained by beating the optical carrier and the +1 st order sideband of an intensity-modulated signal. A quadrature phase is introduced due to the inherent π/2 phase shift between the optical carrier and the +1 st order sideband of the phase-modulated signal. The generation of a 4.5 GHz microwave signal at 625 Msym/s with quadrature phase-shift-keying (QPSK) modulation and the transmission of the signal over a 25-km single-mode fiber are evaluated. An error-free transmission is achieved at a received optical power of 6 dBm. By incorporating I/ Q imbalance compensation, an error-free transmission is achieved at a lower received power of 2 dBm.