Because of its multi-size and marking miscellaneous, the dimension mark of complex combination is difficult, especially for the location dimension. Based on shape-breakdown system, the basic idea of three-basis method is to obtain the location dimension of other bodies relative to the datum body in the combination by defining the datum body, datum lines and datum points. The special advantage of this method on dimension mark has been proved through theoretical analysis and examples interpret. Moreover, it gives a new connotation in dimensioning the combination efficiently and exactly.