Slope analysis method was applied to determine the stoichiometry of the solvent extraction reaction of Ni(II) by Alamine336 from strong HCl solution range from 1 to 10 M. Solvent extraction data was obtained from the literature. The effective equilibrium constant for the solvent extraction reaction was estimated by considering the ionic equilibria of $NiCl_2$ in the HCl solution. The measured distribution coefficients of Ni(II) agreed well with those calculated in this study. Our results suggest that further study on the measurement of the activities of nickel complexes at high HCl solution need to be done.