The temperature dependence of ac susceptibility of YBCO bulk samples was measured as a function of ac field amplitude and frequency. Analysis of the temperature dependence of the ac susceptibility near the transition temperature (T c) has been done employing the simplified Kim model. We have obtained an empirical function for the penetration field H p = H α(1−t)β, t = T=T c. Best fitting to data was obtained with parameters H α ≈ 6:2 × 103 A/m and β ≈ 1.50. The experimental value agrees well with the model calculations. In addition, as the frequency increases, the peak temperature (T p) shifts to higher temperature. This effect can be interpreted in terms of flux creep. The field dependence of activation energy obtained from the Arrhenius plots for the frequency (f) and (T p) can be described as U ∝ (H ac)−β′ with β′ ≈ 0:38 for YBCO.