A field study was conducted to assess the effect of natural and mechanical ventilation systems on air contaminants of poultry housing under eastern province of Saudi Arabia climatic conditions. The concentration and particles size distribution of airborne particles and gases were measured and analyzed inside two different poultry housing. The obtained data showed that, the mean total suspended particles concentration (TSP) inside the poultry housing that used natural ventilation system (N.V.) was 12.47 mg/m³, the particulate matter with a diameter less than or equal to 10 µm (PM₁₀) concentration was 4.81 mg/m³, and the particulate matter with a diameter less than or equal to 2.5 µm (PM_2.5) concentration was 0.18 mg/m³. Whereas, in the poultry housing used mechanical ventilation system (M.V.), these concentrations, respectively, were 4.61, 2.26, and 0.09 mg/m³. The TSP values were greater than the suggested threshold values for indoor air contaminants in livestock building, however, the PM_2.5 values of both housing did not exceed the suggested threshold values for indoor air contaminants in livestock building. The geometric mean diameter (GMD) based on the mass concentration of particles in the natural and mechanical ventilation systems was 8.63 and 8.38 μm, respectively. The concentration of ammonia (NH₃), carbon dioxide (CO₂), sulfur dioxide (SO₂), oxides of nitrogen (NO₂), and hydrogen sulfide (H₂S) was also measured inside the poultry housing. The obtained results also revealed that, the ammonia was the dominant gas in both housing. Moreover, the majority of gases did not exceed the threshold values. Using the mechanical ventilation system, the concentration of airborne particles and toxic gases inside the poultry house was strongly affected by the barn ventilation rate.