A simple closed-form formula is obtained for the transition probability of a two-state atom in a Gaussian laser pulse. The validity of the formula is tested by comparisonwith numerical results. The analytic formula is applicable over a wide range of the laserintensities, detunings and pulse durations. Particularly, it can be used for calculation ofexcitation probability when the Rabi frequency is of the order of or larger than a frequencyassociated with the pulse duration.