Hydrate blockages can occur in natural gas pipelines operated under high pressure and moderately low temperature conditions. Accordingly, the current study was carried out to understand the mechanism of hydrate plugging and examine inhibitors so as to prevent hydrate plugging in natural gas pipelines. The experiments were performed using a flow loop apparatus consisting of a 1.575 cm inside diameter and 400 cm long tube with a 20 cm long transparent view pipe. Most experiments were conducted at 4.90 MPa, 5.90 MPa, and 6.87 MPa with various flow velocities ranging from 0.28 through 0.78 m/s. The temperature in the loop was decreased at a rate of 2.2 K/h from initial temperature of 290.15 K until the pipe was plugged by hydrates. The hydrate equilibrium conditions under various flowing conditions with the methane were measured. And, the hydrate forming phenomena were observed when adding methanol as an inhibitor. The results of both the plugging tendency and the effect of the methanol concentration are presented. Consequently, the current study provides methods for predicting hydrate plugging phenomena and hydrate control techniques using an inhibitor for use in sub-sea pipeline systems as well as PNG (pipelined natural gas) transmission systems.