For natural gas pipelines, it is important to monitor and locate corrosion incidents along the long-distance pipelines. This is especially true for internal corrosion, as it is more challenging to access the inside of the pipes. Distributed optical fiber-based sensors (DOFS) provide unique advantages for natural gas pipeline monitoring. These advantages include long-distance distributed monitoring, flexibility, small size, and improved safety compared to electrical-based sensors in flammable gases. This makes DOFS promising for early corrosion detection. Previously, we have demonstrated DOFS are capable of humidity/water and corrosion sensing inside pressurized tubing in the lab. Here, we report the pilot-scale demonstration of DOFS inside an operating high-pressure natural gas pipeline for distributed measurements along the pipe. Using an optical backscatter reflectometer, the multi-parameter DOFS was demonstrated in a natural gasflowing pipeline (up to 1000 psi, 15 ft/sec flow rate) for monitoring humidity, pressure, temperature, etc. Compared to most studies of DOFS outside the pipe, this is the first demonstration of DOFS inside a natural gas pipeline.