X-ray diffraction by a one-dimensional gradient crystal with the regular variations of the lattice metrics caused by the variation of the chemical composition along the growth direction is considered. The diffraction curves calculated within the suggested model reliably describe the observed asymmetric profiles of reflections from the growth face of a gradient K_1-xRb_xC₈H₅O₄ crystal (x = 0-0.25). It is shown that the gradients of the parameter b obtained in modeling and calculated based on the data of direct measurements are in good agreement.