We studied the ionization probability of Na atoms in an intense Ti–sapphire laser at 800 nm versus the laser pulse duration. The ionization probabilities for the Na(3s) and Na(3p) states were found to vary non-monotonically with the pulse duration at a constant peak laser intensity. The abnormal (non-monotonic) pulse duration dependence was traced to the competition between resonant and non-resonant multiphoton ionization processes, which occurs when the frequency broadening due to the finite pulse duration is compatible with the energy difference between the excitation energy and multiphoton energy.