A simple and accurate delay model is proposed for Ultra Deep Sub-micron CMOS circuits (CMOS Inverter, NAND2, NOR2 etc) based on nth power law when the channel length is less than the 90nm. All the parameters are taken from BSIM.4.6.1 manual. This work derives analytical expression for the delay model of a CMOS inverter including all sorts of secondary effects i.e. Body Bias effect, Channel Length Modulation Effect (CLM), Velocity Saturation effect, Drain Induced Barrier Lowering (DIBL), Gate Induced Drain Leakage (GIDL), Substrate Current Induced Body Effect (SCBE), Drain-Induced Threshold Shift (DITS), which may occur in the Ultra Deep Submicron MOS devices. We also extend our delay model for 2 inputs CMOS NAND & NOR gates. Our result is better than simulation result with respect to both quality and estimation time. This work also thoroughly described the delay dependence on different parameters such as channel length, Load Capacitance, Supply voltage, Transition time, Velocity Saturation Coefficient, Threshold Voltage, Load Resistance, Width variation etc. This also explained the different power models for the estimation of UDSM circuts.