This paper presents the study of time domain inverse scattering for a two-dimensional inhomogeneous dielectric cylinder buried in a slab medium via the finite difference time domain (FDTD) method and the asynchronous particle swarm optimization (APSO) method. For the forward scattering part, the FDTD method was employed to calculate the scattered E fields. Base on the scattering fields, these inverse scattering problems were transformed into optimization problems. The APSO method was applied to reconstruct the permittivity of the two-dimensional inhomogeneous dielectric cylinder. In addition, the effects of Gaussian noise on the reconstruction results were investigated. Numerical results show that even when the measured scattered fields were contaminated with Gaussian noise, APSO was able to yield good reconstructed quality.