A novel oligomer based on alternating fumaric acid and poly(ethylene glycol) (PEG) units was synthesized and characterized. End group analysis by nuclear magnetic resonance (NMR) spectroscopy showed that the oligo (PEG fumarate) (OPF) had end PEG chains and multiple fumarate groups along its macromolecular chain. According to thermal characterization by differential scanning calorimetry (DSC), the crystallinity of OFF was found to be lower than PEG. The molecular weight and the degree of oligomerization of OFF determined by gel permeation chromatography (GPC) revealed that the conversion of OFF was substantially affected by the steric factors associated with large PEG chains. OFF was cross-linked using radical polymerization in the presence of either a chemical initiator or photoinitiator. As the OFF cross-linked, the unsaturated fumarate bonds disappeared and the PEG proton peak broadened in the NMR spectra. The cross-linked OFF gel showed swelling behavior that was dependent on the molecular weight of PEG. This novel cross-linkable PEG macromer is the first macromer with unsaturated double bonds along its macromolecular chain that allows for the preparation of hydrogels with tailored structure and properties.