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In this study, CFD model was developed to investigate the effect of roof vent opening on natural ventilation of fully open-roof multi-span greenhouses. Using the Particle Image Velocimetry (PIV) data (Lee et al., 2002), the CFD validity was preferentially investigated. The results showed that the internal airflow of the CFD were very similar with the PIV computed airflow patterns. Considering the averaged air velocity vertically under the middle gutter of 2-span greenhouse, the errors of the CFD computed results were 33.6, 18.2, and 13.5 %, respectively for the external wind speeds of 3.5, 4.4, and 6.3m/s at gutter height. It was assumed that the 2-dimensional input of the wind profile in the CFD model might cause the positive error compared to the PIV computed results. As the external wind speed increased, the strong incoming airflow stream moved from right to left at the second roof vent. As the number of the span increased over six, the error became mostly less than 10%.