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We are proposing the extraction of necessary electrical parameters of graphene (Gr) on Si/SiO2/Gr and Si/SiO2/Al2O3/Gr heterostructure THz modulators using THz measurement technique. The obtained average THz absorption is 24.5% more on Si/SiO2/Al2O3/Gr as compared to the Gr on Si/SiO2. The calculated values of the carrier mobility of graphene on Si/SiO2/Al2O3 is 2.33 times more than that on Si/SiO2. Presence of Al2O3 may play a role of barrier for diffusion of trap and impurity charges from Si/SiO2 to graphene which may lead to higher mobility and higher THz absorption. THz modulation measurements by optical pumping were also performed. Maximum modulation depth was 18.54% on Si/SiO2/Al2O3/Gr modulator at 2W pumping power which is 16.54% higher as compared to Gr on Si/SiO2. This shows that graphene on Si/SiO2/Al2O3 heterostructure exhibits great potential for the development of an efficient electro-optical THz modulator as compared to Si/SiO2/Gr modulator.