Abstract: The furnace tubes are the vital components in direct fired heater. The wall thickness of the tube has direct relations to the safe operation of heater. The thickness measurements had been respectively made for the box-like type and spring type heaters, the results showed that inside the heater, the corrosion on the individual straight tube mainly centred around its two ends which closed to the bends within the range of 0-900 mm, the more serious local corrosion of the tube mainly centred at the 8 corners of the wall, and the 4 among them on the front side of the wall are the most serious. Through analyzing on the corrosion, it is considered that the corrosion is a kind of sour corrosion resulted from the sulphuric-acid molecules in the soot combined with water droplet condensed on the tube. The countermeasures against the corrosion and overhaul program are proposed.