Abstract: Standard method for evaluating the corrosion rate of formatesis not presently available. A method of evaluating the corrosion rate of formates to casing/tubing strings has been presented with an idea borrowed from the methods of evaluating the corrosion rate of metals. In laboratory experiment, metal coupon weight loss method and a CGF-II HTHP static corrosion tester were used to study the influencing factors such as the type of the formate, production technique, pH, steel material quality, density, and retarder etc. It has been found that using TP140 steel coupon, the corrosion rate of sodium formate was more than 2 times that of potassium formate, because the Cl content in sodium formate (0.619%) was greater than the Cl content in potassium formate (0.0243%). For potassium formate, different production processes resulted in different sulfur contents; potassium formate A contained 0.18% sulfur, far more than that of potassium formate B (0.0477%) and potassium formate C (0.046%), and thus, the corrosion rate of potassium formate A was much higher than that of the latter two. The corrosion of formate salts to steel is much severe under acidic conditions. Water solutions of formate salts taken from a well site needed to be buffered with sodium carbonate and sodium bicarbonateor potassium carbonate and potassium bicarbonate. The corrosion rate of TP140 steel in potassium formate solution was 20 times of the steel BG13Cr or JFE13Cr, indicating that potassium formate had lower corrosion rate to stainless steel than to carbon steel. Since high concentration of HCOO^- is good for the protection of steel, the corrosion rate of TP140 steel in 1.20 g/cm³ potassium formate solution was more than 2 times of the corrosion rate of TP140 in potassium formation solution of 1.40-1.57 g/cm³. Thus, corrosion inhibitors should be used when low density formate muds are used. Based on these experimental data, the use of corrosion inhibition measures can be justifed when using formate drilling fluids.