We present a report on a calculation of scaterring length for I=2 S-wave two-pion system from two-pion wave function. Calculations are made with an RG-improved action for gluons and improved Wilson action for quarks at a-1=1.207(12) GeV on 163×80, 203×80 and 243×80 lattices. We investigate the validity of necessary condition for application of Lüscher's formula through the wave function. We find that the condition is satisfied for lattice volumes L≥3.92 fm for the quark mass range . We also find that the scattering length can be extracted with a smaller statistical error from the wave function than with a time correlation function used in previous studies.