This paper presents the bonding strength between the embedded rebar and substrate concrete by using geopolymer paste as the bonding agent. The determination of the suitable mix proportions of geopolymer paste as bonding agent was the main interest. Twelve different mix proportions of geopolymer paste by varying the amount of starting binder materials and alkaline concentration were prepared and tested for compressive and bonding strengths. The tested results indicated that both RHBA and SF incorporating with FA could be used in preparation of geopolymer paste. Mixes with SF gave the higher compressive and bonding strengths while the mixes with RHBA required the longer curing time. The bonding strengths of round bar and geopolymer pastes were slightly higher than that of control concrete (1.05-1.12 times) and there were significantly high in case of deformed bars (1.03-1.60 times). The ratios of bonding strength on the compressive strength were also presented. In comparison with commercial repair materials, the bonding strengths of geopolymer paste were higher than those of epoxies about 1.24-1.81 times. These tested results indicated that the bonding strengths using geopolymer paste were high enough and possibly used as bonding material for repair works. The mixtures containing high SF content and high NaOH concentrations were recommended to enhance both compressive and bonding strengths.