This paper presents an influence of imperfect saturation on undrained deformation behavior for sand. A torsional shear apparatus is newly modified and capable of undrained shear tests and measurements of S- and P-wave velocities for a saturated soil specimen. S- and P-wave velocities were measured for various partially saturated sand specimens, and P-wave velocity was found to be very sensitive to imperfect saturation. Undrained torsional shear tests were performed at various partially saturated conditions, measuring excess pore water pressure; this decreases significantly as the degree of saturation decreases. Nonlinear deformation characteristics were slightly influenced by B-value at the mean effective stress of 30 kPa.