Abstract: This study employs the Quantum Monte Carlo (QMC) method to calculate positron annihilation lifetimes for monovacancy defects in diamond and monocrystalline silicon, overcoming the empirical approximations inherent in conventional two–component density functional theory. By directly solving the many–body Schrödinger equation through variational and diffusion Monte Carlo approaches, we construct an optimized Jastrow–Slater many–body wavefunction system based on a 64–1 atom supercell model. Results demonstrate: diamond monovacancy annihilation lifetime of 149.4±3.9 ps, closely aligning with the experimental value of 152 ps; silicon calculation yields 287.4±3.6 ps, approaching the experimental range of 270 ps–282 ps and outperforming traditional methods. Electron–positron density distributions reveal localized characteristics at defect centers, validating the method's capability to describe many–body quantum effects. Through elimination of correlation functional approximation errors, this approach achieves quantitative error range control. The research confirms QMC's methodological breakthrough in material defect characterization, establishing a high–precision computational framework for complex systems.