Abstract: The Dumond graphic method is employed in the analysis of crystal curvature radius measurement. A new method for measuring the curvature radius using the (n^v,-n^s) double-crystal diffractometry with Kα₁ spectrum only is proposed. This method is much more sensitive than the usually used (n^s,-n^s) double-crystal diffractometry with the Ka doublet method. The detectable limit of curvature can be improved about one order of magnitude. The curvature radius of the Si crystal after As⁺ ion implantation is measured by (n^v,-n^s) double-crystal diffractometry.