The formation of characteristic layered inclusions in the single crystal of rubidium acid phthalate (RAP) is discussed in view of the effect of its microscopic structure. That the three types of micro structure channels trap the impurities or other different phase precipitates in solution, via a special pair of the conical planes with the highest growth rate and the fluctuation of the growth conditions in liquor, is the main mechanism of inclusion formation. The above argument is in fair agreement with the experimental results for real crystals.