A theoretical model based on rate equations of a four-level atom with a common ground level and three closely spaced upper levels interacting with a standing wave has been developed and solved numerically to simulate the saturated absorption spectrum of rubidium. Doppler-free and collision free Lamb dip Rb-D-2 transitions were recorded by using an external cavity diode laser spectrometer operating at 780 nm. The simulated spectra could reproduce the observed spectrum very well. The value of Rabi-frequency and the saturation parameter could be evaluated. (C) 2004 Elsevier B.V. All rights reserved.