Three grades of liquid urea-formaldehyde (UF) resin with different formaldehyde emission levels such as super E0 (SEO), E0 and El were modified by adding different amounts of microfibrillated cellulose (5 wt% MFC and 95 wt% water) that had been isolated by mechanical disintegration of pulp fibers. Thermal properties of these UF resins were investigated to understand thermal curing and degradation behaviors of the modified UF resins, using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The DSC thermograms showed an exothermic curing reaction, and the curing peak temperature of modified UF resins heavily depended on the emission resin grade with an increasing order from El, E0 to SEO. The addition of MFC suspension into the UF resins gradually increased curing peak temperature suggesting a decrease in the resin reactivity. TGA results showed three main thermal degradation temperatures for the modified UF resins except the SEO UF resin, which had four degradation temperatures.