This paper presents a theoretical investigation to evaluate and compare the capabilities of using dielectrophoresis (DEP) and optical image-driven dielectrophoresis (ODEP) forces for micro/nano scale manipulation. A simplified model of particle velocity as a function of electrode size and particle radius was derived based on Stokes' law. Then, electric field analysis of two typical electrode configurations for DEP and ODEP manipulation was conducted under the same applied AC power and electrode dimension. Our results indicate that compared to DEP, ODEP provides much higher dominant force for micro/nano particle manipulation due to its Gaussian distribution of the light beam. Moreover, with only Brownian effect, theoretical results indicate that the minimum radius of particles that ODEP can manipulate has a close dependence on the virtual electrode size. ODEP can manipulate nano particles down to 2nm when the spot size down to 1μm.