Abstract: We investigated the effect of boric acid on the crystallization of L-lysine monohydrochloride dihydrate on self-assembled monolayer(SAM) of L-cysteine.SAM of L-cysteine was employed to induce the crystallization of L-lysine monohydrochloride dehydrate(LMH).It was found that the crystals of LMH grew preferentially in solution rather than on the SAM.That the SAM did not promote but inhibit the nucleation of LMH was found firstly in the research field of inducing crystals by SAM.This unusual result may largely due to the repulsion between the positive charges of the LMH and L-cysteine.The LMH-boric acid(n(LMH):n(H₃BO₃)=1:1) system was introduced to study further.The XRD results showed that diffraction peaks at 14.9° and 27.9° corresponding to the lattice plane of(101) and(112) appeared first.It means that boric acid prefer to grow on the SAM of L-cysteine.Then the LMH crystallization was improved remarkably after the crystallization of boric acid.After 24 h,we obtained diffraction peaks corresponding to the LMH lattice plane of(101) and(112).The SEM results showed that the appearance of the LMH crystal also changed greatly,from massive on the SAM to needle form on boric acid crystals.The above results demonstrated that boric acid could be used to accelerate the growth speed of LMH and affect its crystal morphology significantly.