To meet the increasing requirement of high-temperature die attach for power module on the excellent thermal conductivity and reliability, a novel Ag paste modified with AlN nanoparticles was designed. The thermal conductivity of the joint sintered by this modified Ag paste during long-term thermal aging was tested. It was found that with 1 wt% addition of AlN nanoparticles, the thermal conductivity of the joint was increased from 164.3 to 273.0 W/(K·m) due to the significant decrement of porosity. But the thermal conductivity of sintered joints was decreased slightly with the excessive addition of AlN nanoparticles. Meanwhile, the increasing porosity and pore size were observed in all joints during the following thermal aging and hence the thermal conductivity was decreased. But the pore growth in the joint sintered by Ag paste consisting of 1.0 wt% AlN nanoparticles was quite limited, which still had a high thermal conductivity of 249.4 W/(K·m) after 1008 h thermal aging, showing great thermal conductivity and reliability.