We successfully demonstrated a low-loss, flat-passband, and athermal arrayed-waveguide grating (AWG) multi/demultiplexer with a Mach-Zehnder interferometer (MZI) as an input router. Resin-filled trenches were formed in the longer arm of the MZI as well as the slab in the AWG to compensate for the temperature dependence. A 32-channel athermal multi/demultiplexer was fabricated using silica-based planar lightwave circuit (PLC) technology. A small temperature-dependent wavelength shift of 0.02 nm was obtained over the temperature range of -5 to 65 degrees C with low-loss (3.3-3.7 dB) and flat-passband spectra.