Based on a nonlinear least squares optimisation, a deconvolution method is proposed to remove the system resolution from the ion implantation profiles measured. The method is applied to the range distributions of 200 keV Hg ions implanted into nickel at various tilt angles. The results show that the range straggling is markedly reduced after the deconvolution, the skewness and kurtosis are slightly larger than those of the experimental measured profiles, and they all can be represented by a Pearson IV distribution that is different from the Pearson I distribution predicted by TRIM. The longitudinal and lateral range straggling for normal incidence could be deduced from the angular dependence of the deconvoluted range straggling.