Gradient nonlinearity causes systematic errors in the measurement of DTI parameters. These errors can be greatly reduced if the actual fields generated by the gradient coils is known. Although the gradient field maps are known to the manufacturers, many users do not have access to them. We describe a method for measuring the gradient field maps using a set of diffusion weighted images of an isotropic diffusion PVP phantom. We use the field maps to analyze a DTI study of the phantom and compare the results to analysis performed without the field maps. The results show that the method works well.