This article presents a novel and robust approach for implementing ultra-low power MOS current mode logic (MCML) circuits. To operate at very low bias currents, a simple and compact high resistance load device has been introduced. Operating in subthreshold regime, the circuit can be used in a very wide frequency range by adjusting the bias current without any need for resizing the devices. Measurements in 0.18 mum CMOS technology show that the proposed MCML circuit can be operated reliably with bias currents as low as 1 nA offering a significant improvement of the power-delay product compared to conventional CMOS gates. Simulations show that the proposed circuit exhibits faster response compared to the conventional MCML circuits with triode-mode PMOS load devices at low bias currents.