We measured temperature dependence of elastic modulus for the high-quality single crystals of Y1-xCaxTiO3 (x=0,0.1 and 0.2) using an ultrasonic technique. A step like elastic anomaly due to the volume magnetostriction induced by a ferromagnetic transition was observed in longitudinal C11 and C33 for x=0 and 0.1, although no anomaly was observed for x=0.2. With increasing x, the magnitude of anomaly decreases, reflecting that the amplitude of the antiferromagnetic orbital order parameter, where the orbital ordering is the origin of the ferromagnetism of YTiO 3 , is reduced with increasing Ca concentration.