Polymethyl methacrylate (PMMA) thin films were prepared by spin coating using chloroform solvent. The field emission current versus applied field characteristics were tested. Stable field emission was observed after a switch-on process. The field emission properties of PMMA film exhibit vacuum gap dependence characteristics. The scanning electron microscope inspection reveals that after emission process microwhiskers were formed at the surface. The gap dependence of the field emission properties was explained. It is proposed that the high field during the switch-on process leads to the formation of the microwhiskers. (c) 2007 American Vacuum Society.