In the branching analysis by size exclusion chromatography (SEC)/triple detection (concentration, light scattering, and viscosity detectors) method, the branch number is calculated from the extent of chain size contraction due to chain branching relative to the linear polymer of the same molecular weight (MW). A problem can arise from the fact that polymer chain size depends on both MW and chain branching. Since SEC separates polymers according to the chain size, an SEC fraction of randomly branched polymers may contain polymer species heterogeneous in both MW and chain architecture in general. As a solution of the problem, we propose a separation of polymers by interaction chromatography according to MW first and then measure the chain size distribution of polymers in the homogeneous MW fraction by SEC/triple detection. The analysis scheme is successfully established by online two-dimensional liquid chromatography combining temperature gradient interaction chromatography and SEC/triple detection.