Type III bursts are one of the early discovered forms of solar radio emission and observed in the impulsive phase of solar flares. The characteristics of these bursts are that these are emitted on the fundamental and second harmonics mode. It is believed 1) that fundamental plasma emission occurs through the decay L → T+S of a Langmuir wave L into an ion-sound wave S and a transverse electromagnetic wave T . Second harmonics emission occurs through the coalescence process L + L′ → T of two Langmuir waves, L and L′, to produce a transverse electromagnetic wave T . Recently Gyoubu et al. 2) showed by using a 2-D EM PIC code that electromagnetic waves (R-mode) can be emitted from Langmuir waves generated by the electron beam (velocity:0.5 ∼ 0.7c) instability. They concluded that the second harmonics electromagnetic waves can be generated by a new nonlinear process, called the plasma maser. In the recent Type III observation, Poquerusse 3) reported that electron beam speed causing related to electromagnetic wave emission is close to 0.8c, which will lead us to examine the possible effect of the relativistic electron beam (Lorentz factor: γ = 1.7). In the present paper, we investigate by using a 2-D EM PIC code, whether electromagnetic waves on the fundamental and second harmonics mode can be generated from such a relativistic electron beam.