We have measured the resonance line profile and the Rabi oscillation in the 6s(2) S-1(0) - 6s6p P-3(1) transition of an ytterbium atom pumped by a pulsed, single mode laser. By fitting the Rabi oscillation curve with a phase diffusion model, we could obtain a more accurate value of the oscillator strength of the transition. We showed that the phase diffusing field and the Gaussian temporal shape of a single mode laser dampens the Rabi oscillation curve. They also shift the peak positions of the curve.