Abstract: Currently the application of 50 drill pipe is more in the standard penetration test (SPT) in the geotechnical engineering investigation, which is different from the code that 42 drill pipe is specified. Based on the project in Xiaoyi of Shanxi province, the results of SPT with drill pipes of 50 and 42 are compared, and the effects of diameters on the number N of SPT are statistically analyzed. The results show that the diameter of drill pipes has a certain influence on the number N of SPT, and the effects are different due to different test depths and strata. When the test depth is from 0.0 to 30.0 m, the ratio of the average values of number N of 42 to 50 drill pipe is between 0.85~0.87, and the ratio is 1.09 when the test depth is from 30.0 to 40.0 m. In silty clay and silt, the number N of SPT measured by 50 drill pipe is greater than that by 42 drill pipe, while in dense sand, the number N of the two kinds of drill pipe is close.