Barium sulfate (BaSO4) scale deposition is a serious problem encountered during the secondary oil recovery process. Many scale inhibitors are currently used to prevent the scale formation. Therefore, this research focused on studying the effect of testing time, scale inhibitor concentration, initial solution pH, and type of scale inhibitors on the formation of BaSO4 precipitates. The scale inhibitors used in this work were Aminotri(methylene phosphonic acid) (ATMP), Diethylenetriamine--penta(methylene phosphonic acid) (DTPMP), and Phosphinopolycarboxylic and polymer (PPCA). The concept of a critical supersaturation ratio was developed to characterize the effectiveness of the scale inhibitors on the BaSO4 scale inhibition. The critical supersaturation ratio at which the BaSO4 precipitaion occurs was obtained at different testing times and used as an index to evaluate the effect of various precipitating conditions on BaSO4 scale inhibition. The results indicated that the critical supersaturation ratios decreased with increasing testing time until craching a constant value, but increased with increasing scale inhibitor concentration and initial solution pH. Higher scale inhibitor concentration and initial solution pH resulted in smaller and more spherical BaSO4 particles. The results also revealed that a longer testing time, a higher scale inhibitor concentration, a higher initial solution pH, and a greater number of ionizable protons gave a broader particle size distribution and a smaller mean diameter of the BaSO4 precipitate. PPCA was found to be more effective for BaSO4 inhibition than DTPMP or ATMP.