Free-space optical (FSO) communication systems are where free space acts as a communication channel between transceivers that are line-ofsight (LOS) for successful transmission of optical signals. FSO communication are a promising candidate to support the high-speed connections requirements of the fifth-generation (5G) and beyond wireless communication networks. The FSO transmissions through the atmosphere, nevertheless, bring significant challenges, besides the uncertainty of atmospheric channels, especially the signal fading due to the atmospheric turbulence, attenuation and pointing errors caused by the random beam misalignments between transceivers, signal obstruction due to buildings or trees can prevent the transmitted message to reach the destination. In this paper, we theoretically analyze the average symbol error rate (ASER) of reconfigurable intelligent surfaces (RIS) assisted FSO link over log-normal turbulence channels. We study the RIS effect by taking into account the influence of link distance, transmitted optical power and quadrature amplitude modulation (QAM) scheme on the ASER.