This paper examined the effect of grain surface roughness on phase transformation line and failure line under undrained torsional simple shear. To begin with, the term “normalized maximum void ratio e*_max” was defined as a measure of grain surface roughness. Then, monotonic loading test, stress control liquefaction test of constant amplitude and strain control liquefaction test with gradual increase in strain amplitude were carried out using sands of the same gradation with different surface roughness. Finally, the relationships between e*_max and phase transformation line were discussed. The test results suggest that the effects of microscopic dilatancy originated from grain surface roughness are significant on undrained shear behavior.