La in La0.67Ca0.33MnO3 has been partially substituted by Dy and Yb. With increasing doping amounts of Dy and Yb, the Curie temperature decreases monotonically. When La is substituted by Dy, the transition temperature decreases monotonically, the peak resistivity increases rapidly, while the magnetoresistance ratio (MR) increases dramatically. The substitution of 13% (atomic fraction) Dy for La increases the MR ratio by almost 40 times. The influence of doping Yb on the MR is very small. The mechanism of colossal MR (CMR) can be explained in terms of spin clusters.