The performance of free-space optical communication (FSO) with the combined effects from scintillation effect and pointing errors under Gamma-Gamma atmospheric turbulence is investigated. Assuming FSO system uses intensity-modulation/direct detection (IM/DD) with on-off keying (OOK), the novel closed-form expressions of bit error rate (BER) and outage probability for FSO system are derived. How the performance is affected by the atmospheric turbulence and other parameters such as the normalized beamwidth, the average transmitted optical power, the normalized jitter, signal-to-noise ratio is researched. Numerical simulation is further provided to verify the derived analytical expressions. The results show that optimizing the beamwidth can achieve the minimum BER for a given average transmitted optical power.