Positron lifetime spectroscopy was used for studying various oxygen-deficient YBa 2 Cu 3 O 7 - δ (δ = 0.06 - 0.68) at normal state (300 K) and superconducting state (77 K). Using the two-state trapping model, the experimental results were analysed and there exists a typical positron annihilation characteristics in this system. The local electron densityn e and vacancy concentration C v are calculated as a function of oxygen-deficiency δ. The mechanism of positron annihilation and the correlation with superconductivity are also discussed.