We discuss detailed characteristics of a polarization-division-multiplexed quadrature-phase-shift-keying (PDM-QPSK) modulator with a hybrid configuration of silica planar lightwave circuits (PLCs) and LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (LN) phase modulators. With a polarization multiplexing circuit consisting of a half-wavelength plate (HWP) and a Mach-Zehnder-type polarization beam combiner (PBC) in a PLC, the modulator outputs a PDM-QPSK signal with a inter-polarization-channel crosstalk smaller than -40 dB. The modulator also has a small insertion loss of 5.0 dB (7.7 and 8.4 dB for each polarization), and successfully operates at 100 Gb/s.