Spontaneous decay behavior from an assembly of atoms (or molecules) in a dielectric slab sandwiched by two dielectric semi-infinite spaces with arbitrary refractive indices is investigated. We introduce a lifetime distribution function to describe decay kinetics. The zero-point fluctuations of the electromagnetic field are calculated as a function of the position of atom in the slab. They display a symmetric profile with respect to the central plane of the slab in symmetric structure sample and the contributions of the guided modes and the radiation modes components both exhibit cosinelike variation with a base but with a phase difference of p. With the increase of the thickness of the slab, the oscillations become dense. In contrast, for the asymmetric structure case, the related curves are asymmetric with respect to the middle plane of slab and exhibit considerably distorted cosinelike oscillations. The dependence of the spontaneous emission rate of atoms on the thickness of the slab and the refractive indices of substrate also is surveyed. Our calculations show that quite wide or narrow lifetime distribution can occur for different thicknesses of the slab. The coexistence of both accelerated and inhibited decay processes appears. The calculated results are in good agreement with experimental results.