The proper viscosity of urea-formaldehyde (UF) resin adhesive for optimum adhesion depends on the type of a raw wood material for wood-based composite panels. This study investigated the practical relationship between apparent viscosity and molecular weight (MW) of UF resin adhesives during the control of 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 of 100:0, 75:25, 50:50, 25:75, and 0:100 to obtain different viscosities. The 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, allowing the prediction of the MW of UF resins based on their apparent viscosity after synthesis. The weight average molecular weight (Mw) values fit well with 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 alpha, 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 resin adhesives.