Oxygen uptake rate (OUR) was used as an indicator of microbial activity. In this study OUR at dyeing wastewater in the pilot plant was monitored to examine biological activity. Correlation between inlet COD concentration and maximum OUR showed that maximum OUR was proportional to inlet COD concentration. Changes in the OUR values reflected the changing waste load in the reactor. Consequently, OUR can be used to estimate biological activity of inlet COD concentration. This study showed that biodegradable COD at dyeing wastewater could be calculated from OUR and yield coefficient. Non-biodegradable COD was able to be calculated from a difference between initial COD concentration and biodegradable COD.