One of the important features of the microstructure of yttria-sintered AlN ceramics is the second phase. The type, morphology, and distribution of the second phase influence the thermal conductivity. In the present research, the effects of sintering temperature, time and cooling rate on the second phase evolution were investigated. Different morphologies of second phase were detected, characterized and quantified using scanning electron microscopy and image analysis. Moreover, the thermal conductivity of the specimens representing different microstructurcs was measured. The major second phase present in the specimens was YAM. The second phase was present in isolated, interconnected, agglomerated morphologies the volume fractions of which change significantly by varying the sintering parameters. The total quantity of second phase(s) increases when the sintering temperature is increased. The direct effect of second phase morphology on the thermal conductivity of AlN is discussed.