The notable effect of atomic radius ratio of the components of L12-0rdered alloys on the temperature dependence of their strength has been demonstrated using suitable alloys and alloying elements. Pt3 In and Pt3Sn were chosen and partial substitutions were made of the minor components with Ga and TI, and with Ge and Pb, respectively, keeping the electron concentration of the system constant. The positive temperature dependence was generally found to be strengthened by the smaller substituting atoms and to be weakened by the larger atoms. On the other hand, the substitution of In was found to strengthen the anomalous behaviour of Pt3 Ga. Thus, a maximum anomalous strengthening is achieved with an In-rich Pt3(I n, Ga) alloy, which forms a continuous solid solution with a L12 structure. All these effects are consistent with the model obtained previously by experiments on binary L12 alloys and are correlated to the radiusratio rule, which leads to the change of phase stability of L12 alloys.