The use of high strength line pipe steels is beneficial for the reduction the cost of gas transmission pipelines by enabling high pressure transmission of large volumes of gas. The high strength line pipe steels will become the preferred materials for modern natural gas transmission pipeline. It was well known that manganese was an important element in the high strength line pipe steels. In this paper, a simple spectrophotometric method was described for determination of manganese in high strength line pipe steels. The method was based on the oxidation-reduction reaction between ammonium persulfate and manganese(II) producing manganese(VII) in the presence of silver nitrate as a catalyst. The characteristic wavelength of maximum absorption of manganese(VII) was obtained locating at 530 nm. Under the optimum reaction conditions the absorption value was proportional to the concentration of manganese in the range of 0.18%~2.0% (R2 = 0.9997), and the relative standard deviation was less than 3.0% (n=5). The proposed method was applied successfully to determine manganese in API grade X80 line pipe steel and API grade X70 line pipe steel samples.