While the parametric curve is overwhelmingly used in current character motion animation practice, the utilization of the implicit curve is recognized to be extremely potential for motion animation since the implicit surface have succeeded in application such as modeling, deformation and rendering. In this paper, we investigate the use of implicit curves in motion animation. Implicit curves for task specification in motion animation is proposed and further explored in this paper. An implicit curve is used to specify the motion path while a speed profile curve is adopted to describe the motion timing. Then an approach and algorithm to motion inbetweening along implicit curves is proposed. Based on the motion path and motion speed, inbetweening technique is utilized to generate inbetween motion sequences from which skeleton sequences for character animation are obtained by inverse kinematical technique.