The chain length distribution of a oligomeric polystyrene sample initiated by n-butyl lithium was determined by high performance size exclusion chromatography on a ultraviolet absorption detector. The effect of the end group on the weight fraction of the individual mono-disperse species both for the experimental UV response data and the theoretical distribution function was discussed. An end group correction factor was introduced for getting the true weight distribution of chain length. The experimental data could be precisely represented by superimposition of two poisson distributions. The deviation from a single poisson distribution is believed as a consequence of the variation of the mode of mixing of the polymerizing reac-tants.