In this paper, experimental investigation of pressure exponent in burning rate of composite propellant was conducted. Four sets of different propellant compositions had been prepared with the combination of Ammonium Perchlorate (AP) as an oxidizer, Aluminum (Al) as fuel and Hydroxy-Terminated Polybutadiene (HTPB) as fuel and binder. For each mixture, HTPB binder was fixed at 15% and cured with isophorone diisocyanate (IPDI). By varying AP and Al, the effect of oxidizer-fuel mixture ratio (O/F) on the whole propellant can be determined. The propellant strands were manufactured using compression molded method and burnt in a strand burner using wire technique over a range of pressure from 1atm to 31atm. The results obtained shows that the pressure exponent n, increases with increasing O/F. The highest pressure exponent achieved was 0.561 for propellant p80 which has O/F ratio of 80/20.