A theoretical analysis of the lasing gain and the threshold current in GaAs-AlGaAs Graded-Index Separate Confinement Hetero (GRIN-SCH) structure lasers is given. Energy levels and the state densities for electron distribution in both the well layer and the optical guiding regions are formulated. Confinement ratios of the injected electrons and holes and the optical field in the well layer are examined. The injection current, the electron lifetime due to the radiative recombination, the lasing gain and the threshold current are examined by taking into account the intraband relaxation effect of the electron wave. Optimum conditions to obtain low threshold current density are numerically discussed.