Excitation of Tm3+ to H-3(4) using the 791 mm pump source showed the frequency up-converted blue emission (similar to 480 nm) due to the Tm3+:(1)G(4) --> H-3(6) transition in Tm3+/Nd3+ codoped CaO . Al2O3 glasses. Intensity and Lifetime changes with rare-earth concentrations suggested the efficient energy transfer of Tm3+:H-3(4) --> Nd3+:F-4(5/2) and Nd3+:F-4(3/2) --> Tm3+:(1)G(4). The latter transfer enabled Tm3+ to reach its (1)G(4) level, and the blue emission became possible through the (1)G(4) --> H-3(6) transition. Quantitative analysis with rate equations proved that these two transitions were the most efficient among all the possible energy transfer routes between Tm3+ and Nd3+. Calculated up-conversion efficiency of the Tm3+/Nd3+ combination in CaO . Al2O3, glass was 6.6 x 10(-3), and it was similar to4 orders of magnitude larger than those reported for other oxide glasses.