In petrochemical processes, it is necessary to ensure the reliability and validity of process data for admimstrative management and many technical applications. These include process control and online srmulation and optimization. Steady-state onlme data reconciliation and new methods for data classifying, steady-state identification and gross error handling were adopted to develop an online data reconciliation system (DRS). The DRS has bee successfully implemented with the online optimization system in the crude oil distillation unit at Lanzhou petroleum refinery in China Using reliable and valid data achieved through online data reconciliation, the online optimization results showed a profit increase of more than 4 million RMB.