Fin effectiveness and efficiency of a thermally asymmetric rectangular fin are represented as a function of non-dimensional fin length, width, fip tip surface Biot number and the ratio of fin bottom surface Biot number to top surface Biot number. For this analysis, three dimensional separation of variables method is used. One of the results shows that fin effectiveness can be increased or decreased depending on the fin length as the fin tip surface Biot number increases while fin efficiency decreases without depending on that as the fin tip surface Biot number increases.