The laser fusion criterion is known as the ρR-Criterion, also called high-gain condition. This parameter is temperature dependent and can be calculated by R-matrix method. This method is applied for determining improved fusion cross-section for the reactions T(d,n)⁴He, ³He(d,p)⁴He, D(d,p)T, D(d,n)³He. In this paper the time dependent reaction rate equations for fusion reaction T(d,n)⁴He are solved and by using the obtained results we computed the fusion power density, energy gain versus temperature and ρR-parameter. The obtained results show that a suitable combination may be a deuterium fraction f_D=0.65 and f_T=0.35 which would lead 30% reduction in the tritium content of the fuel mixture, and this choice would not change the energy gain value very much. Finally, the obtained energy gain for D-T reaction by using R-matrix is in good agreement with other theories.