In this paper, the influences of fiber link and laser source on performances of Radio over Fiber (RoF) including error vector magnitude (EVM), constellation and eye diagram are investigated by simulation using Opti-system12 (trial version). The investigated RoF network is built on IEEE 802.16a WiMAX, with 16 QAM and a Mach-Zehnder modulator for intensity modulation. The mechanism of that the dispersion in fiber link makes the constellation rotate is investigated. The relationship between the rotation angle of the constellation and dispersion is analyzed, where we first put forward a fitting formula to describe this approximate quantitative relation. In the analysis of the influence of the laser source on the network, where the dispersion compensates fiber (DCF) is applied to compensate the rotation in constellation caused by fiber link, the threshold in the relationship curve of the linewidth and EVM is obtained. It is found that if the laser linewidth exceeds this threshold, the EVM will increase rapidly, then, the performance decreases.