Glasses co-doped with Tm3+/Yb3+ can emit red(Tm3+ ; 1G4longrightarrow3F4) and blue ((Tm3+ ; 1G4longrightarrow3H6) radiations via upconversion process. Efficiencies and mechanism of the upconversion process strongly depend on the wavelength used for the excitation. Current study is concerned with the blue upconversion mechanism in calcium aluminate glasses co-doped with Tm3+/Yb3+ when the 791 nm wavelength pump source was used to excite Tm3- directly to its 3H4 level. Spectroscopic analyses and the lifetime measurements suggested the following energy transfer mechanism. Tm3+ was excited to the 3H4 level. by absorbing 791nm pump radiation and ten transfer the energy to Yb3+ via Tm3+:3H4longrightarrowYb3+ : 2F5/2. Then the energy released through the Yb3+:2F5/2longrightarrow 2F7/2 transition was re-absorbed by Tm3- to excite it from the 3H4 to 1G4 level. Blue upconversion with the wavelength of 480 nm occurs from the transition of Tm3+:1G4longrightarrow3H6 Optimum composition for the with the wavelength of 480nm occurs from the transition of Tm3+:1G4longrightarrow3H6 Optimum composition for the most efficient upconversion was found from the glass with 0.3 mol% Tm2O3 and 0.5mol% Yb2O3