Abstract: In this paper, relative brightness profiles along plasma tail axis and several sections perpendicular to the axis for comet Halley on 4 January 1986 are analysed with the aid of Hayar-chuk's theoretical diffusion model and other relevant data. By fitting observed brightness profiles with theoretical profiles, some of physical parameters for the plasma tail are estimated, such as the mean lifetime of CO⁺ ions, longitudinal (along tail axis) and transverse diffusion coefficients, possible lower and upper limits of magnetic field intensity in direction of the tail axis. Finally, certain conclutions and discussions are made briefly.