This study was designed to observe the effect of melatonin on haematological parameters in buffalo calves under summer stress. Twelve healthy Murrah buffalo male calves of 6 month to 1 year age group were taken for the study. Buffalo calves were divided into control (CG) and treatment (TG) group. In TG, Melatonin (18mg/50 kg BW) was injected subcutaneously, two times at 1st and 20th day. Blood samples were collected on 0th, 7th, 14th, 21st, 28th, 35th and, 42nd days and analysed by using hematology analyzer. Between the days, in CG the monocytes count, granulocytes count, monocytes percentage, granulocytes percentage, RBC(1012/L) count, HGB(g/dl), HCT(%), MCV(fL), MCH(Pg), RDWCV(%), PLT(109/L) count, MPV(fL), PCT(%) and in TG granulocytes percentage, HGB(g/dl), HCT(%), MCH(Pg), RDWCV(%), RDWSD(fL) was observed to vary significantly (p<0.05) at some time points. Between the groups the granulocytes percentage on day 7th and 28th was observed significantly (p<0.05) high in CG as comparison to melatonin TG. The overall effect of granulocytes percentage during 42 days of buffalo calves were found significantly (p<0.05) higher in control group than the melatonin treated group. In conclusion, melatonin affects the haematological parameter to counteract the summer stress condition in animals.