CaMnO3 systems are known to exhibit excellent thermoelectric (TE) performance at high temperature in air and are, therefore, good candidates for the n-type legs of TE oxide devices. In previous studies, however, several CaMnO3 system legs have fractured easily after the power generation test. The fractures were probably caused by the inherently low mechanical strength of CaMnO3, due to which it developed a great thermal stress arising from the big difference in thermal expansion coefficient between the Ag electrode and the CaMnO3 leg. In the present study, Ag particles were dispersed in a Ca0.9Yb0.1MnO3 matrix to mitigate the problems mentioned above. The incorporation of Ag particles into the matrix led to both improvement of the mechanical strength of Ca0.9Yb0.1MnO3 and to reduction in the thermal stress at the junction between the Ag electrode and the Ca0.9Yb0.1MnO3/Ag composite leg. Additionally, the ability of TE oxides to withstand application of a maximum temperature gradient without impairment of their mechanical strength could be significantly improved as a result.