API tubing is the devise used to transport oil or natural gas from the hydrocarbon reservoir to the earth surface. Tubes mainly include three different kinds, namely non-upset tubing (NU), external upset tubing (EU) and integral joint tubing (IJ). NU refers to the tube whose ends are threaded and coupling directly without being thickened. EU means both ends of the tube are thickened, threaded and then coupling. IJ refers to the tube whose ends are connected without coupling, one end being threaded through internal thickened and the other through external thickened.

API divides tubing grades into three groups. Group 1 is for all tubing in grades H40, J55, and N80. Group 2 is for restricted-yield tubing grades L80, C90, and T95. Group 3 is for high-strength tubing in seamless grade P110. The API grade letter designation was arbitrarily selected to provide a unique name for various steels. Numbers in the grade designation indicate the minimum yield strength of the steel in thousand psi. API defines the yield strength as the tensile stress required to produce a specific total elongation per unit length on a standard test specimen.

Grade H40 is generally not used in tubing sizes due to the relatively low yield strength. J55, commonly used grade for most wells, has been the "standard" grade for tubing in most relatively shallow (< 9,000 ft) and low-pressure (< 4,000 psi) wells on land. N80, a relatively old grade with open chemical requirements, is susceptible to H2S and acceptable for sweet oil and gas wells in design conditions. L80 is a restricted yield-tubing grade that is available in Type 1, 9Cr, or 13Cr. L80Type 1 is less expensive, but more subjects to weight-loss corrosion, commonly used in oil and gas fields. Despite the popularity for CO2-and mild H2S-contaminated wells in the past, Type 9Cr has largely been supplanted by Type 13Cr. L80 13Cr tubing has become popular for its good CO2-induced weight-loss corrosion resistance properties. C90 is relatively new with two different chemical requirements: Type 1 and Type 2. Only Type 1 is recommended for use in sour service. T95 is a high-strength tubular grade with different chemical requirements: Type 1 and Type 2. Only Type 1 is recommended for sour service. P110, the high-strength tubing, is normally used in deep sweet oil and gas wells under high pressure. This grade is sensitive to SSC failures unless the temperatures are relatively high (> 175°F).

API Tubes are supposed to bear the pressure during the extraction period, which means it should be of great pressure resistance and proper malleability. Besides, it ought to be sized to perfectly satisfy the expected rates of oil and gas products, neither too big nor too small. API 5CT and 5B make specific requirements to the production of tubes, such as tensile strength, yield strength, elongation and hardness, etc.