We have modeled the structure of grain boundaries in high‐Tcsuperconductors. Geometrical modeling coupled with atomic relaxation to minimize the energy illustrates that each repeat block of the boundary consists of coalesced and void‐like regions. The grain boundary voids are insulating, but, tunneling of superconducting pairs through coalesced regions is responsible for critical current density associated with the boundary. The strain field around the boundary is used to evaluate the depression of the order parameter and the transmission coefficient. The length of the region, in which nonstoichiometry from cation and anion point defect distributions extends, is determined from the solution to the Poisson’s equation. The spatial variation of the depression of the order parameter and the scattering from the nonstoichiometric regions are used to determine the inhomogeneity in the critical current density. The critical current density is derived as a function of misorientation angle, temperature, and nonstoichiometry from the present model. The results are found to be in excellent agreement with available experimental data in high‐Tcsuperconductors.