In-situ mullite toughened 3Y-TZP ceramics (Mullite/3Y-TZP ceramics) with different content of mullite were prepared by gel-casting combined with pressureless sintering to study effects of the content of mullite on their microstructure and tribological properties. The results indicated that the mullite uniformly distributed in the zirconia matrix had a complete columnar structure and a moderate content in S2 (the sample containing 4 mol% mullite), and the smooth surface caused by low porosity combined with the good mechanical properties significantly reduced the coefficient of friction and wear rate of S2. A high proportion of irregular mullite destroyed the overall structure of S3-S5 (the samples containing 6, 8 and 10 mol% mullite, respectively), combined with high porosity not only reduced the mechanical properties of them, but also led to an increment in surface roughness, which significantly lowered the wear resistance of the material. Studies of worn surfaces showed that micro-cutting was present in every sample, but the degree of micro-cuffing and adhesive wear in S2 were minimal due to its good structure and properties. However, the micro-cutting and adhesive wear of S3-S5 were exacerbated.