For analysing field data as well as for modelling purposes it is useful toclassify tree species into a few functional types. In this paper a newaggregation of tree species of the dipterocarp rain forests in Sabah(Borneo), Malaysia, is developed. The aggregation is based on the twocriteria successional status and potential maximum height. Three classesof successional status (early, mid and late successional species) and fiveclasses of potential maximum heights (<=5 m, 5-15m, 15-25m, 25-36m, >36 m)lead to a combination of 15 functional types. The criteria of the developedclassication are chosen to suit for applications with process-based models,such as FORMIX3 and FORMIND, which are based on photosynthesis productionas the main process determining tree growth. The concept is universal andcan easily be applied to other areas. With this new method of grouping amore realistic parametrization of process-based rain forest growth modelsseems to be possible.