The backward wave oscillator is one of the most important devices for generating high-power RF. One of the major challenges of backward wave oscillators is their large physical volume, especially at low frequencies. In this paper, a backward wave oscillator based on metamaterials is presented, which is the most important feature of this miniaturization structure. In this paper, the conventional backward wave oscillator and the backward wave oscillator with a metamaterial structure were first examined. It was observed that in a certain frequency band (e.g. S-band), the metamaterial-based backward wave structure has a dimension reduction from 60 mm to 20 mm compared to the conventional structure, which represents a 66% reduction.. Then, a single metamaterial element is used intermittently inside a cylindrical waveguide as the interaction region. An electron beam with a beam voltage of 180 kV and a current of 24 A is used as the lamp excitation, and a magnetic field of 0.5 Tesla is used to prevent electron beam divergence. The operating frequency is 2.87 GHz and The peak power efficiency is obtained to be 64%, considering the maximum output power of 2.76MW.