A new theoretical approach for the prediction of the growth habit of crystals is presented. This approach is based on a newly derived relation between the growth rate of crystal surfaces and habit-controlling factors, and includes a key step: a so-called interface structure (IS) analysis. This analysis is to formulate the influence of the fluid phase on the crystal morphology. The essential of the IS analysis is to identify the adsorbed growth units which is in dynamic equilibrium with solid units at the crystal surface, and to calculate their concentration. It follows that a key external habit-controlling factor, the so-called surface scaling factor, can be calculated from the analysis. Based on detailed molecular dynamic (MD) simulation data, our formalism is applied to predict the morphology of urea crystals grown from aqueous solutions. Urea crystals grown from the solutions turn out to possess a needlelike shape, in excellent agreement with experiments. This is one of the first examples of the successful theoretical prediction of morphology of crystals, and will provide a new way of thinking and understanding of the influence of the mother phase on crystal habits.