We presented a method for calculating the chemisorption energy of simple gases on transition metals series. The simplified d-band model of Allan has been applied to describe the electronic structure of transition metal surface, and the chemisorption energy is calculated by making use of the generalized phase shifts. The method is employed to study the adsorption of hydrogen and oxygen on 3d and 4d transition metals. The calculated results of the chemisorption energies show remarkable systematic trends, in good agreement with experiment. The calculated adsorbate induced electronic energy level [also being referred to as the split-off energy] is at approximately 6 eV below Fermi energy, that is comparable to UPS experiments. The conclusion deduced from the calculation is as follows: the difference between the Fermi level and effective adsorbed energy level (EF-εa.) plays the crucial role in determining the trends in the chemisorption energy, the variation in the bandwidth across the series and the adatom-metal atom transfer matrix element ν are of secondary importance.