We investigated 260 quadrats (50cm long x canal width) in three small agricultural canals to examine physical and hydraulic conditions of mussel (Order: Unionoida) habitat. Total abundance (N/m2) of four mussel species were significantly higher in quadrats that the dominant bed materials were sand, gravel, and pebble, than in quadrats that concrete or mud were dominant. Multiple regression analyses showed positive relationships between mussel abundance and three environmental variables: sediment depth, total ratio of sand, gravel, and pebble, and current velocity. These results indicate the importance of hydraulic conditions, which sand, gravel, and pebble are maintained without mud deposition, for mussel habitat in agricultural canals. We also examined stability of sediments in the study canals by comparing shear stress of quadrats and critical shear stress of sand (2 mm), gravel (16 mm), and pebble (64 mm). In the result, the study canals generally had hydraulic conditions which gravel and pebble are stable even at high water level (shear stress: <11.37 cm/s) and sand is somewhat unstable even at ordinary water level (shear stress: 3.94 ± 1.76 cm/s [avg. ± SD]). To maintain mussel habitat in agricultural canals, shear stress at high water level should be less than the critical shear stress of gravel in maximum size (16 mm).