In this study, an objective-based two-dimensional (2D) surface plasmon (SP)-enhanced optical trapping system with a spatial light modulator (SLM) has been developed to trap dielectric particles in freeform pattern. Through a gold film with a thickness of 45 nm in the near infrared region, a 40-fold electric field enhancement is reached and hence a strong 2D trapping force distribution with SP excitation has been demonstrated. Furthermore, the algorithm called weighted Gerchberg-Saxton can provide a freeform pattern which is used to control the trapping force distributions in the image space based on SLM. Unlike the patterns formed by finite gold areas fabricated on a glass surface, the freeform plasmonic trapping is a more convenient and efficient method to manipulate nanoparticles and biomolecules arbitrarily. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).