Numerical simulations of sympathetic detonation in gap test were carried out by Eulerian hydrodynamic code. In this computer code, a less diffusive and stable algorithm, CIP method, has been adopted, and the Ignition and Growth reactive flow model was added to solve shock initiation phenomena. Both of the donor and the acceptor charges were Composition B, and gap material was PMMA. The set up of gap test was varied to understand the size effect. We found that the critical gap length and the charge diameter have linear relationship with logarithmic scale.