The Friis formula provides an efficient method for system noise figure (NF) evaluation. Nowadays, many additional features are required for the RF front-end designs such as compact size, low cost, and low power consumption. RF system on chip (SoC) is a solution, since in most of the SoC designs, for an optimized performance, the impedances between different RF stages are not matched at fixed real impedance. This paper will discuss the application of Friis formula to the SoC designs, and discuss possible solutions for cascaded NF calculation for the RF front-end designs with non-standard matching impedances.