Epitaxial a-axis oriented YBa2Cu3O7-delta (YBCO) superconducting thin films have been grown by off-axis magnetron sputtering on Y2O3/YSZ/Si substrates with PrBa2Cu3O7-delta (PBCO) seed layer. The YBCO thin films were deposited immediately after the on-axis magnetron sputtering of PBCO. XRD analyses show that the a-axis volume fraction for 120 nm thick YBCO films varies with substrate temperature during PBCO deposition and its maximum value is higher than 98%. The TEM study shows the clear dependence between the character of the R-T dependence and the microstructure of our YBCO thin films, which varies with the change of the volume ratio of a-axis to c-axis oriented YBCO.