According to the law of diminishing marginal product, if the units of variable factor keep on increasing while keeping the level of fixed factor constant, then at initial stages the marginal product will rise but finally, a point will be attained after which the marginal product of the variable factor will gradually fall. After this point, the marginal product of any additional variable factor will be zero and can even be less than zero.  The tendency of the marginal product to first increase and then fall is called the law of variable proportions or the law of diminishing marginal product.