The variation of intensity of the spectral lines of the triplet series of mercury namely (73S1 → 63P012) (λ5461 Å,λ 4358 Å,λ 4047 Å) in the presence of a longitudinal magnetic field between zero and 2000 gauss has been investigated. The effect of magnetic field is found to be different as regards the variation of intensity and the occurrence of maxima in the three lines. These variations can be explained by considering the reabsorption of the spectral lines. A mathematical theory has been presented and an expression for (Iul)B/Iul has been deduced where (Iul)B and (Iul) are the intensities of the lines with and without the magnetic field. The experimental results agree fairly well with the theoretical model.