In situ grown Y-Ba-Cu-O films were prepared on MgO substrate by rf magnetron sputtering with dc-biased substrate holder using a stoichiometric YBa2Cu3Oz target. The films were deposited at substrate temperature Ts about 660 and various substrate holder bias voltages VH. Influence of a dc bias voltage of substrate holder on the superconducting characteristics of Y-Ba-Cu-O thin films are investigated. Films with critical temperature higher than 80 K can be obtained at bias voltages from 20 V to 80 V for 500 mTorr. The optimum bias voltage to obtain the high Tc films depends on the sputtering gas pressure. The optimum bias voltages in 500 mTorr and 300 mTorr are 20 V and 150 V, respectively. The film morphology depends on the bias voltage rather than the sputtering gas pressure in this experiment and the smooth film surface is obtained at low voltages from 20 V to 40 V. For the films deposited at 80 V in 500 mTorr, the film of 200 nm thick is smoother than films of 400 nm and 700 nm thick. The deviation of the film composition is varied by the negative holder bias voltage. The critical temperature depends on the c-lattice parameter rather than the film composition. The films with short c-lattice parameter have the high Tc. The negative holder bias is effective for obtaining the high Tc films in the low pressure. However, in the high pressure, the films at the high bias voltages have slightly low Tc as compared with films at low bias voltage.