An investigation of the thermal drag phenomena is conducted both analytically and experimentally. The existence of thermal drag gives rise to the decrease of both the mass flow rate and Mach number in the flow systems. Defining the thermal drag coefficient Ct and dimensionless heating number He, an approximate correlation between them is obtained for one-dimensional, inviseid duct flow with simple heating. The dependence of the ratio of friction-caused pressure drop ΔPf to heating-caused ΔPt, on the heating number is presented for turbulent duct flow, and clarifies the relative importance of the thermal drag to the viscous drag for different heating intensity.