An extensive observation has been made on the external form of cassiterites of various genetic types. The properties of their crystal faces are investigated by using PBC (periodic bond chain) theory and, as a result, three kinds of F faces are recognized, i. e., s{111}, m{110) and e{101}, in which s{111} has failed to be recognized by P. Hartman himself from rutile with the same crystal structural type. The forms of cassiterite have been drawn both through the measurement of natural crystals and through the deduction from its crystal structure, with the two results quite in agreement with each other.