The radical polymerization behavior of ethyl ortho-formyl-phenyl fumarate (EFPF) using dimethyl 2,2’-azobisisobutyrate (MAIB) as initiator was studied in benzene kinetically and ESR spectroscopically. The polymerization rate (R(p)) at 60 degrees C was given by R(p) = k[MAIB](0.76)[EFPF](0.56). The number-average molecular weight of poly(EFPF) was in the range of 1600-2900. EFPF was also easily photopolymerized at room temperature without any photosensitizer probably because of the photosensitivity of the formyl group of monomer. Analysis of H-1- and C-13-NMR spectra of the resulting polymer revealed that the radical polymerization of EFPF proceeds in a complicated manner involving vinyl addition and intramolecular hydrogen-abstraction. The polymerization system was found to involve ESR-observable poly(EFPF) radicals under the actual polymerization conditions. ESR-determined rate constant (2.4-4.0 L/mol s) of propagation at 60 degrees C increased with decreasing monomer concentration, which is mainly responsible for the observed low dependency of R(p) on the EFPF concentration. Copolymerizations of EFPF with some vinyl monomers were also examined.