The present paper is aimed at studying the reflection of thermoelastic waves under the generalized thermoelasticity theory is employed to study the reflection of plane harmonic waves from a semi-infinite elastic solid under the effect of the magnetic field in a vacuum. The formulation is applied under the thermoelasticity theory with three-phase-lag. The expressions for the reflection coefficients, which are the relations of the amplitudes of the reflected waves to the amplitude of the incident waves, are obtained. Similarly, the reflection coefficient ratio variations with the angle of incident under different conditions are shown graphically. Comparisons are made with the results predicted by different theories Lord-Shulman theory (L-S), the Green-Naghdi theory of type III (G-N III) and the three-phase-lag model (3PHL) in the absence and presence of a magnetic field. The new results obtained clear that the external parameters and theories applied have strong impact on the phenomenon and applicable on the related topics as geophysics, acoustics, aerospace. astronomy, …, etc