Energy confinement properties were compared for D-shaped and circular-cross-section plasmas with an identical horizontal minor radius as functions of plasma current, electron density, and vertical elongation under a wide range of discharge conditions. The property of gross energy confinement can be explained by the combination of an electron energy confinement proportional to eq*(∝(eBT/Ip)(1+K2)/2) and neoclassical ion energy confinement proportional to The high-current operation capability of D-shaped plasmas produces an improvement in energy confinement at high density due to the reduction of neoclassical heat loss in the ions. The highest energy confinement time (75 ms) is obtained for high-current and high-density D-shaped discharges.