Systematic studies on the optimization of depositing superconducting YBCO thin films on Si substrates with YSZ buffer layers by pulsed laser ablation have been carried out. Nearly c-axis oriented YBCO films with Tco ≈ 85K were obtained routinely on Si(100) substrates with in-situ deposition of YSZ layers on top. Preliminary measurements on the critical current density Jc of these films showed Jc > 2.0 × 104 A/cm2 at 77K and Jc> 1.4 × 106 A/cm2 at 20K. The significant improvements on Jc are attributed to the great reduction of microcracks and the crystallinity of YSZ layer, achieved by the in-situ annealing of the buffer layer prior to the YBCO deposition. The effects of oxygen partial pressure and substrate temperature during YSZ deposition on the crystallinity of YSZ layer as well as on the quality of subsequently deposited YBCO films will be discussed. In our most recent attempt the microcracks has been sucessfully eliminated by a process combining the in-situ annealing and slow cooling process.