This study investigated the relationship between apparent viscosity and molecular weight of UF resins in their synthesis. The UF resins were synthesized at various formaldehyde/urea (F/U) mole ratios ranging from 0.8 to 1.5 with different apparent viscosities. In addition, low- and high-viscosity UF resins with 1.0 and 1.2 F/U mole ratios, respectively, were mixed at five different blending ratios 100:0, 75:25, 50:50, 25:75, and 0:100 to obtain different viscosities. The molecular weight (MW) of each resin was measured by gel permeation chromatography, and the relationship between apparent viscosity and MW was derived using the Mark-Houwink (M-H) equation. The results showed a good relationship between the two parameters. The weight average molecular weight (Mw) values fit well to the M-H equation, while the number average molecular weight (Mn) values did not. For the first time, this paper has reported that k and α, constants of the M-H equation based on Mw of the UF resin, ranged from 0.015 to 0.017 and 1.172 to 1.276, respectively. These suggest that the relationship between apparent viscosity and Mw should be considered for the synthesis of UF resins