Quantum cascade lasers (QCLs) are promising high output power semiconductor-based terahertz (THz) sources with narrow linewidths and wide operating frequency. We have grown and fabricated THz-QCLs based on GaAs/AlGaAs superlattices under the guidance of a newly developed simulation tool based on the framework of Non-Equilibrium Green's Function (NEGF) method. The latest THz-QCLs demonstrated 270 mW peak power and 2.4 mW average power at 10 K, which is comparable to the highest reported value but with a size of more than 4 times larger than ours. Temperature dependence of the device performance are analyzed through the detailed calculations by using the NEGF method.