Abstract: The incomplete isomorphism is a special type of isomorphous replacement for REE. An incomplete isomorphous REE fractionation models are presented herein. In these models, fourteen elements of REE are postulated to be considered as one element, whereas the others as isotopes. These elements can be fractionated in geological processes. According to these models, the fractionation values of each REE can be calculated with respect to chondrite-normalized values. The formula for calculation is δREE=(REE/ΣREE)_sample/(REE/ΣREE)_chondrite-1×100. The modal diagrams then may be constructed by using the calculated values as y-coordinate and atomic numbers as x-coordinate. The modal diagrams are well referred to as the incomplete isomorphous REE fractionation models cited above, and will improve our knowledge of petrogenesis, especially of the origin and evolution of magmatic rocks.