Lubricant base oils were prepared from two Fischer-Tropsch (Fr) waxes and two alpha-olefins with different carbon number distributions under several isomerization/hydrocracking conditions. The molecular structures of the resulting oils were investigated using C-13 NMR analysis to determine the location and length of branches. Peak areas assigned to the CH carbons were divided into eight groups and correlated with the progress of the isomerization reaction. Each group showed good correlation with the density of branching, which was expressed as the ratio of the average branching number (ABN) to the average carbon number (ACN). This trend was independent of the feedstock used and the reaction conditions. The probability of methyl branching at a carbon atom depended on its location from the terminal carbon; that is, in order of decreasing probability, the carbon location is second > third > fourth, and so forth, and the probabihty of the seventh and eighth or inner carbon atoms was almost equal. A trend of increasing proportion of branches located at the second carbon was observed. Diesel oils were also obtained by isomerization/hydrocracking of FT waxes, and the most likely position of methyl branching was the second carbon from the terminal carbon. Branching at the second carbon showed a decreasing trend with increasing density of branching in diesel oil, whereas that in lubricant base oil showed an increasing trend. The present work demonstrated that the position and length of the branches in lubricant base oils, and diesel oils prepared by isomerization/hydrocracking of FT waxes and alpha-olefins, are determined by the density of branching, thus supporting previous findings that viscosity properties of lubricant oils, such as the kinematic viscosity and viscosity index, can be expressed using only the ACN and ABN.