Abstract: Experiments were conducted in the high pressure distillation column containing corrugated structured packing of Mellapak 250Y under operating pressure from 0.3 MPa to 2 MPa, using n -butane and n -pentane mixture as the separating material. The height equivalent to a theoretical plate (HETP) was evaluated from the measured n -butane liquid concentration profile along the packing. After carefully analyzed, the HETP may be reasonably divided into two parts. The first part is the ideal height of mass transfer unit, which is caused by the mass transfer resistance between vapor and liquid. Another part is the height of backmixing unit (HBU), which is caused by the backmixing of vapor and liquid. The parameters of HBU correlation were evaluated by the experimental data. The present HETP correlation was used to predict the experimental results in literature, and well agreement was found between them.