Quantum cascade lasers (QCLs) in terahertz frequency emit terahertz radiation by intersubband optical transitions in conduction band of semiconductor quantum wells. Terahertz QCLs are the terahertz sources which are able to be electrically pumped and operated in continuous-wave. Terahertz quantum cascade lasers are small, light, compact, and easy for integration, and they are the solid-state terahertz sources that have the highest output power. This article will review the recent developments of double-metal terahertz QCLs in terms of narrow beam pattern, frequency tunability,and high output power.