X-Ray fluorescence cross-sections (sigma(Li), i = alpha, beta, l, gamma) of the Cs-55 element were calculated theoretically at excitation energies for each L, (i = 1, 2 and 3) subshell, respectively. Coster-Kronig transitions (f(12), f(23) and f(13)) are non-radiative transitions in which an inner shell vacancy is transferred from one subshell of an atom to another. The increase in L X-ray intensity due to the effect the Coster-Kronig transitions on L X-ray fluorescence cross sections were calculated theoretically. These calculated values were compared with other experimental and theoretical values. Calculations showed that the alteration in absolute intensities of L-upsilon and L-rho lines arising from Coster-Kronig transitions are greater than that of L-beta.