Abstract: There exist inevitably model errors in the adjustment function models of the earth's gravity field,due to the various systematic errors.The quality of adjustment results in some extent depends on the errors of the function models.For this purpose,the errors of the function models have to be compensated or controlled to the greatest extent.The optimum seeking criteria of the adjusted function models are given in this paper.Then the criteria for judging the function model errors are presented,as well as their practical estimation methods.Furthermore,the formulae for evaluating the fitting standard deviation of the function models are given.