Abstract: AIM：To investigate the effect concentration of provitamin D (pro D) in ethanol on the yield of its photoisomers. METHODS: The pro D solutions in ethanol at different concentrations were irradiated by using two different ultraviolet light sources. The concentrations of photoisomers were quantitatively determined through a normal phase HPLC. RESULTS: The data indicated that, with increasing concentration of pro D, the yield of tachysterol (T) decreased, while that of lumiserol (L) obviously increased. The yield for previtamin D (pre D), however, was light source dependent. CONCLUSION: The effect of concentration of pro D on the yield of its photoisomers is totally unexpected, particularly for T and L, due to self-photoabsorbing effect. By use of the self-photoabsorbing effect, these results can not only be satisfactorily explained; but also a theoretical basis is provided to design some useful parameters in photochemical drug synthesis like vitamin D, such as the depth of a photoreactor, the optimum original concentration of pro D, or with the purpose of preparing a certain photoisomer with high yield.