Cooperative communication systems are wireless communication systems in which user/users communicates to a destination via relay/relays. In recent years, cooperative communication has a wide interest. However, most studies do not address the power allocation part. In this paper, the effect of power allocation in cooperative transmission technique using decode and forward (DF) protocol based on virtual noise is investigated. For performance evaluation, optimum power allocation (OPA) coefficients are generated and the results of OPA are compared with equal power allocation (EPA) results for different path loss scenarios. Both theoretical and simulation results show that power allocation implementation provides better bit error rate (BER) of the proposed system. In this system using binary phase shift keying (BPSK) modulation, it is shown that full diversity gain is achieved with numerical and theoretical results obtained over Rayleigh fading channels.