By systematically varying the oxygen content of microstrip line resonators made of pulsed laser deposited YBa2Cu3O7-x films, the effect of oxygen deficiency on the effective penetration depth were studied. By using the two-fluid model, the zero-temperature penetration depths for the fuIly oxygenated (i.e. O=7) and oxygen deficient (O=6.8) states in the same strip are found to be 483.2 nm and 789.8 nm, respectively. The large values obtained are attributed to the granular nature of the YBCO films used. The correlation between the penetration depths and superconducting carrier densities as functions of the oxygen content are also discussed.