Rheology of carbon black (CB) suspensions in an alkyd resin‐type varnish (Varnish‐2), a rosin‐modified phenol resin‐type varnish (Varnish‐1), and a polystyrene/di‐butyl phthalate (PS/DBP) solution was investigated to clarify the effects of CB morphology such as primary particle size and DBP absorption value (a measure of aggregate structure). It was found that the important parameters to characterize the CB aggregates are the effective volume fraction φeff of CB aggregates evaluated by plotting the relative viscosity ηr = η0/ηm (ηm: medium viscosity) on the universal ηr versus φ curve obtained for the hard‐core silica particles for CB/Varnish‐2 and CB/(PS/DBP) systems, and the critical gel concentration φcrit found for CB/Varnish‐1 systems. Because the φeff and φcrit values depended on DBP absorption value, irrespective of the primary particle size, and were found to be larger for the higher‐structure CB with higher DBP absorption value.