1.2μm - laser pumped wide frequency tunable coherent terahertz (THz) light source is demonstrated. The operation principle is based on difference frequency generation (DFG) with an excitation of phonon - polariton in Gallium Selenide (GaSe) crystal. The pump and signal lasers used are 1.2μm Cr: Forsterite lasers. The tuning range of the THz wave frequency covers from 0.3THz to 4.8THz (type eoo phase matching) and 8.3THz to 10.2THz (type eoo phase matching) under collinear phase matching conditions. It is shown that the maximum conversion efficiency is up to ∼10-6, which is 3 orders in magnitude larger than that of Gallium Phosphide (GaP) crystal.