Over the past two decades, a considerable amount of work has been done on zirconia-toughened alumina (ZTA) to take advantage of the recognized toughening effect induced by ZrO2. In fabricating customized or complex-shaped ZTA parts, conventional manufacturing processes, including slip casting and powder metallurgy, are regarded as time-consuming and cost-intensive. In response to these problems, directed energy deposition (DED) has been proposed and utilized to fabricate customized ZTA parts with highly flexible features in a shorter cycle time at a lower cost. Investigations have been reported on studying effects of input variables (such as laser power) in DED of ZTA parts, however, there are very limited investigations on effects of the ZrO2 content. In this investigation, the effects of the ZrO2 content on microstructures and mechanical properties of DED-fabricated ZTA parts are studied. Experimental results show that at lower levels of ZrO2 contents (5 wt%, 10 wt%, and 20 wt%), a novel three-dimensional quasi-continuous network (3DQCN) microstructure is tailored, whereas at higher levels of ZrO2 contents (30 wt%, 35 wt%, and 41.5 wt%), eutectic microstructure dominates the whole part. Both the 3DQCN microstructure and the eutectic microstructure are beneficial for toughening ZTA parts. In addition, the 3DQCN microstructure contributes to hardening ZTA parts.