The nonlinearity coefficient of microstructured optical fibers (MOFs) is a quantity of great importance in devices based on the nonlinearity of the fibers. The distribution asymmetry of the cladding air holes can make an impact on it. The effect of hetero-diameters of the cladding air holes on the nonlinearity coefficient of a highly nonlinear MOFs with a hexagonal lattice is analyzed by the full-vector finite element method and the error theory. The dependence of the nonlinearity coefficients of the HEx11 and HEy11 modes of the fiber on the diameter of cladding air hole is computed. Subsequently, the dependence of the standard error of the nonlinearity coefficients on the standard error of the diameters of the air holes is obtained. As a result, if the standard error of the diameters of the air holes is given, it is easy to obtain the standard error of the nonlinearity coefficients. When the ratio of the air hole diameter standard error to the designed diameter is 5%, the ratios of the nonlinearity coefficient standard errors of the HEx11 and HEy11 modes to the nonlinearity coefficient of the fiber with uniform cladding air holes are 2.55% and 3.06%, respectively.