The elastic constants of YBa2Cu3O7-delta have been computed employing the method of long waves and unscreened rigid ion model of Chaplot. The acoustic velocities and elastic modulii have been calculated at various temperatures taking the temperature variation of lattice parameters in the quasiharmonic approximation. Good agreement exists between the available experimental results and calculated values. The anomalies in sound velocities are shown to be due to the anomalous variation in the lattice parameter c in some samples.