A full retention of the tetragonal phase with coarse grains $(50~60;{mu}m)$ was possible with the specimen $ZrO_2-1.9;mol\%;Y_2O_3$. In these coarse grains, ${101}_t$ annealing twins were frequently observed, although they do not exist in the usual fine grained specimens. The morphology and growth rate of the isothermally formed individual products are studied at an optical microscopic level. The habit planes of both products are also identified by performing two-surface trace analysis on the grains whose orientations are determined by the Electron Back Scattering Pattern (EBSP) method. The morphologies of isothermal martensite were well-defined thin plates and lenticular types. The growth rate in their longitudinal directions was quite slow and temperature-dependent. A two-surface trace analysis, incorporated with the EBSP method, identified the habit planes near ${013}_c$, in agreement with previous reports obtained from TEM works.