In this paper, a Ka-band Monolithic Microwave Integrated Circuit (MMIC) power amplifier (PA) is reported. Simulation results show that the designed PA using Gallium Nitride (GaN) technology exhibits maximum small signal gain S 21 of 16.7 dB, maximum output power of 41 dBm, and maximum power added efficiency (PAE) of 27 % over operating frequency band of 27-31 GHz with the quiescent bias point of $\mathrm{V}_{\mathrm{D}}=28\mathrm{V}$ and $\mathrm{I}_{\mathrm{D}\mathrm{Q}}=91\mathrm{m}\mathrm{A}$. A very high small signal gain of 27 dB can be achieved if the bias current $\mathrm{I}_{\mathrm{D}\mathrm{Q}}$ is increased to 270 mA. Besides, various stabilizing techniques have been used to make the designed PA unconditionally stable in the whole frequency band from DC to 60 GHz regardless bias conditions. The Ka-band PA has a small chip size of $2980\mathrm{x}2250\mu \mathrm{m}^{2}$.