  We report on a lattice QCD calculation of the I=2 $\pi\pi$ scattering length
using the overlap fermion formulation for both sea and valence quarks. We
investigate the consistency of the lattice data with the prediction of the
next-to-next-to-leading order chiral perturbation theory after correcting
finite volume effects. The calculation is performed on gauge ensembles of
two-flavor QCD generated by the JLQCD collaboration on a $16^3\times 32$
lattice at a lattice spacing $\sim$ 0.12 fm.
