PURPOSE: To economically produce the crystal by adding effective amounts of two compatible components to a hot Bayer process caustic pregnant solution immediately before the crystallization of alumina trihydrate from the pregnant solution.

CONSTITUTION: In this production, in order to reduce the percentage of alumina trihydrate crystals which have small size and are formed during crystallization of alumina trihydrate from a hot Bayer process caustic pregnant solution and accordingly, to increase the yield of coarse alumina trihydrate crystals to be subjected later to treatment for producing aluminum, the following process is adopted. That is, the process involves adding effective amounts of two compatible components A and B to the caustic pregnant solution after the separation of red mud and immediately before the crystallization of alumina trihydrate, wherein: the component A is a surfactant for dispersing the component B in the caustic pregnant solution and as this surfactant, tall oil fatty acid is preferred; and the component B is an oil into which the component A is to be dissolved and has a boiling point higher than the predominant temp. in the crystallization stage of alumina trihydrate and as this oil, a 10C alcohol distillation residue is preferred.