Multipactor, the well-known electron avalanche phenomena occurring in waveguides when the amplitude of the RF electric field exceeds a threshold, is modelled for lower hybrid current drive (LHCD) antennas. The electric field threshold departures significantly from the expected linear scaling with the frequency when the width of the waveguide along the electric field increases. For 5 GHz waveguides, the multipactor does not occur when this width exceeds 26 mm, in very good agreement with analytical formulation. Effect of static magnetic field with typical values of tokamak environment is examined. It is found that the threshold is weakly reduced (−6%) for the front face of the antenna featuring the rows of narrow waveguides. Strong reduction in the threshold is found where the wave frequency equals the electron cyclotron frequency (0.178 T at 5 GHz) but this condition will not be met on ITER. After baking at 240 °C, wide waveguides at the antenna input (29 mm on ITER) have a threshold of ∼7 kV cm−1, assuming a homogeneous magnetic field. However, preliminary results indicate higher threshold when the gradient of the magnetic field along the waveguide is taken into account and no limitation, from this section, is expected for the ITER LHCD antenna.