Abstract: The improved principal component analysis (PCA) was introduced to the monitoring of mould processes during slab continuous casting. Based on analyzing the measured mould friction data, simulation results show that the improved PCA can avoid the conservation of Q statistical test and effectively identify the change in the Hotelling T² test which may caused by process fault or variation in operating condition. Compared with the conventional PCA, the improved PCA is more effective and sensible in fault diagnosis and process monitoring.