Nowadays, due to the high level of competitiveness of the market some companies have sought to find a better administrative and operational efficiency. The technique of statistical process control (SPC) is one the statistical tools that allows a better view of abnormalities in the process and consequent improvement in the quality and productivity of the companies. Developed and widely applied in the production lines of manufactured goods, the SPC has shown a great potential for use in the optimization process of waste treatment. In this context, the present study aimed to characterize the physicochemical properties of the treated effluent from the oilseed agroindustry and analyze the variability of the wastewater treatment process using Shewhart control letters interpreted trough eight criteria of non-randomness proposed by the ISO 8258:1991. The samples collected and the analytical procedures were performed by reference methodologies for the analysis of water and waste water. The statistical analysis consisted in the codification of the variables, Dixon test application and preparation of control letters. The results showed that 10 parameters are out of statistical control of the process, which has shown a consistent variability with five of the eight criteria for detecting special causes, however 9 variables were under statistical process control. In this perspective we note the instability of the process in the effluent treatment of the oilseed agroindustry, which can be corrected with actions that minimize the occurrence of special causes and anticipates the decision to be made regarding the treatment of effluents.