An immersed boundary method for efficient calculation of momentum forcing is presented. Momentum forced equation consists of Navier-Stokes equation part and momentum forcing part. Navier-Stokes equation part is generally discretized using three points, but derivative terms which compose momentum forcing part are discretized using boundary points. Taylor series expansion is considered to add boundary points to discretizing first and second derivative terms. By using neighbor-hood points included boundary points a spatial second-order accuracy is retained. And when momentum forcing part is discretized, n+1 step velocity is used implicitly. So any process to calculate a momentum forcing is not needed. In results, decaying vortex problem is simulated to show a spatial second-order accuracy.
