SUSY(Supersymmetry) is motivated to solve the notable hierarchy problem in SM(Standard Model) and to exploit all the symmetry compatible to the S-matrix. Unexpectedly, moreover, it also provides inkling for GUT(Grand Unified Theory) by showing gauge coupling unification at high energy scale compatible to the Plank scale. We, here, review Supersymmetry theory and it’s minimal extension so-called MSSM to the SM and RGEs(Renormalization Group Equation) of the MSSM parameters. We also present the Seesaw model and study the possibility of quark-lepton mixing angle unification in MSSM at seesaw-GUT scale which is motivated by quark-lepton unification in GUT, in addition to the gauge coupling unification.