Microstructures of 3 mol% yttria-stabilized zirconia (3YSZ) with different content of TiN were investigated in this study by XRD, SEM, and TEM. The specimens were hot-pressing sintered at 1450 degrees C for 2.5 h under 50 MPa in vacuum, and then further characterized on microstructure change and phase distribution. The TiN added to 3YSZ can suppress tetragonal (t) to monoclinic (m)-3YSZ phase transformations, reduce 3YSZ grain size, and improve its mechanical properties. The ceramic was formed of homogeneously dispersed 3YSZ and TiN grains with average size of 0.7 and 1 mu m, respectively. The coexistence of t- and m-phases was observed in one grain, and the nucleation of m-phase with acicular twin occurred preferentially at grain boundaries of t- phase. The TEM analysis was focused on the 3YSZ-30 wt% TiN specimens in order to identify the relationships between t- and m-structures, and twin structures of m-phase. Two crystallographic relations of t- to m-transformation are (100)(m)//(110)(t), [010](m)//[001](t) in type I, and (100)(m)//(110)(t), [001](m)//[001](t) in the type II. The acicular (100)(m) and lathlike (110)(m) twins in m-phase were also observed.