Abstract: The mass transfer characteristics in an annular pulsed extraction column were studied with four different annular widths. 30% TBP(in kerosene)-HNO3 - H2O was used as experimental system (water as continuous phase). The solute concentration profiles under steady state were measured. Mass transfer parameters were estimated from the diffusion model by fitting the measured concentration profiles. Correlation of mass transfer parameters (Hoxand Hoxd) was suggested. Results showed that the "true" mass transfer unit height Hox of four annular extraction columns can be correlated with the same equation and the diffusion unit height Hoxd is in direct proportion to the 0. 16 power of the ratio of annular width to column diameter. In comparison with annular column 0. 05m in diameter, the correlation equation of Hox is the same for two kinds of annular column, but that of Hoxd is different. It indicates that axial miximg is affected by column diameter.