Feline body mass index (fBMI), BW/PCL, length from top of patella to end of calcaneus, was developed as a new diagnostic tool for obesity in cats. To evaluate the effectiveness of fBMI for obese cats in short-term, 6 cats were induced weight gain by over-feeding with high fat diet and then they were induced weight reduction by restrict-feeding with low fat diet to measure changes in fBMI and plasma metabolite concentrations and enzyme activities. BCS 3 is correlated with fBMI 24.6-32.0, BCS 4 is correlated with fBMI 33.1-37.1 and BCS 5 is correlated with fBMI 29.9-40.3, respectively. On the correlation coefficient analysis by Pearson's method (P < 0.05), positive correlations (r = 0.403) were seen between the fBMI and plasma triglyceride (TG) levels. From these findings, fBMI seems to be more sensitive and useful indicator for obesity diagnosis in cats.