Sarfraz [1-] has described and analysed a C1 interpolatory rational cubic function. This rational cubic function involve two free parameters and does not preserve the shape of the positive data. This paper-develop necessary and sufficient conditions under which the positive interpolation is successfully achieved. Moreover, user has a freedom to play further with the curve until a desired shape is achieved. A simple case of constraint interpolation where the curve is forced to lie on the same side of a given straight line is also discussed.