In order to study three-dimensional unsteady flow inside HL240 rotor under design condition, sliding mesh was applied to investigate rotor/stator interaction influence. Coupled flow through guide vane, runner, and draft tube was simulated by using Fluent software, N-S equations, standard k-ε turbulence model and SIMPLEC method. Computational result agreed well with practice. The pressure fluctuation near the runner blade surface, periodic pressure variation inside flow passage, and back-flow in draft tube were revealed through unsteady flow analysis. 