Abstract: Background and Objective: Phosphorus is one of the most important nutrients as a form of phosphate ion in the aquatic environment. The high concentration of phosphate caused the algae bloom. The objective of this study is to determine phosphate uptake by microalgae Oscillatoria sp. and influenced of optical density, salinity and concentration of the microalgae. Material and Methods: Technique diffusive gradient in thin film was used to measure phosphate concentration in solution and it related to phosphate uptake by microalgae Oscillatoria sp. using the spectrophotometric method. Results: The phosphate concentration diffused in the DGT unit related to phosphate uptake in microalgae Oscillatoria sp. The linear correlation between phosphate in DGT unit and phosphate uptake in Oscillatoria sp. with the salinity of 0, 15 and 30 g L–1 resulted in R2 was 0.9820, 0.9449 and 0.9677, respectively. The effect of phosphate concentration by the DGT technique correlated significantly with uptake by Oscillatoria sp. The optical density, algae concentration and salinity were contributed to phosphate bioavailability in Oscillatoria sp. From this study, it can explore the equation to predict of phosphate uptake by Oscillatoria sp. by using [Palga] = {(k×OD)+f)×{PDGT]}. Conclusion: The DGT technique was fairly predictive of bioavailability phosphate to microalgae Oscillatoria sp. Based on the phosphate concentration diffused in the DGT device, it can predict the phosphate concentration in algae cells without the destruction of algae cells. This is the new simple method for measuring bioavailability phosphate in the aquatic environment.