In this letter, the different scattering mechanisms of triphenylene-derived graphene on conventional SiO2/Si substrates and octadecyltrimethoxysilane (OTMS) self-assembled monolayer (SAM) functionalized SiO2/Si substrates were systematically studied at room temperature. In comparison with the devices on conventional SiO2/Si substrates, triphenylene-derived GFETs with OTMS-SAM modified SiO2/Si substrate exhibit the marked carrier-density-dependent field-effect mobility. Quantitative analyses reveal that at ambient temperature, the predominant scattering sources affect the carrier mean free path for graphene devices on bare SiO2 substrates and for those on OTMS passivated SiO2 substrates are charged impurity induced long-range scattering (~5.34 × 10(11) cm(-2) in carrier density) and resonant scattering (short-range scattering ~9.77 × 10(10) cm(-2) carrier in density), respectively. Our findings elucidate the underlying dominant factors for achieving significantly improved device performance of GFETs at room temperature.