Hot carriers injection (HCI) tests for ultra-short channel n-MOSFET devices were studied. The experimental data of short channel devices (75-90 nm), which does not fit formal degradation power law well, will bring severe error in lifetime prediction. This phenomenon usually happens under high drain voltage (Vd) stress condition. A new model was presented to fit the degradation curve better. It was observed that the peak of the substrate current under low drain voltage stress cannot be found in ultra-short channel device. Devices with different channel lengths were studied under different Vd stresses in order to understand the relations between peak of substrate current (Isub) and channel length/stress voltage.