This paper presents a class-E power amplifier (PA) with a novel harmonic tuning circuit (HTC) based on composite right-/left-handed transmission lines (CRLH TLs). One of the issues of conventional harmonically tuned PAs is the limited PAE bandwidth. It is shown by simulation that class-E amplifiers have potential of maintaining high PAE over a wider frequency range than for example class-F amplifiers. To make full use of class-E amplifiers with the superior characteristics, an HTC using double CRLH TL stub structure is proposed. The HTC is not only compact but also enhances the inherently wide operation frequency range of class-E amplifier. A 2-GHz 6W GaN-HEMT class-E PA using the proposed HTC demonstrated a PAE bandwidth (≥65%) of 380MHz with maximum drain efficiency and PAE of 78.5% and 74.0%, respectively.