The corrosion of carbon steel downhole completion tubing at high temperature is one of the serious corrosion issues facing the oil and gas industry. Few corrosion inhibitor chemistries can protect downhole production tubing against corrosion, especially at high-temperature and high-pressure (HTHP) in high total dissolved solids (TDS) brine. The objective of this study was to develop a HTHP corrosion inhibitor abbreviated (NAM 9) for the protection of carbon steel at a temperature of up to 120 oC for possible application in sweet gas wells. The new corrosion inhibitor formulation (NAM 9) was evaluated using the linear polarization resistance (LPR) technique, batch corrosion testing (drop and drip) and autoclave corrosion test. The LPR result shows that 10 ppm of NAM 9 drastically reduces the corrosion rate of C1018 steel from 214.4 mpy (5.44 mm/y) to 3.28 mpy (0.08 mm/y) giving an inhibition efficiency of 98.47 % at 70 oC. The corrosion rate of the blank using autoclave corrosion testing at 120 oC was 250.6 mpy (6.36 mm/y) and 30.3 mpy (0.77 mm/y) in the presence of 500 ppm of NAM 9, respectively. The results indicate that NAM 9 is a promising high-temperature corrosion inhibitor for downhole production tubing used in gas wells.