The study aimed to assess variation of surface water quality and evaluate the monitoring frequence at three continuous monitoring stations on Tien river (MT1 station) and Hau river (MH1 and MH2 station) over a 10-year period (2009-2018) with monitoring frequency of 12 months per year. The water quality variables comprised temperature (^oC), pH, dissolved oxygen (DO, mg/L), total suspended solids (TSS, mg/L), nitrate (N-NO₃^-, mg/L), orthophosphate (P-PO₄^3- mg/L), chemical oxygen demand (COD, mg/L), and Coliforms (MPN/100 mL). Individual water quality parameter was assessed by comparison with National Technical Regulation on surface water quality (QCVN 08-MT: 2015/BTNMT). Monitoring frequency was evalutated using Cluster Analysis (CA). The findings revealed that surface water on Tien and Hau rivers was long-lasting polluted with suspended solids, organic matters, nutrients and microorganisms. The cluster analysis presented that the frequencies of current water monitoring could be reduced from 12 times to 5 times including February, March, June, September and December, resulting in a 58% reduction in monitoring costs. Future study should focus on evaluating water quality parameters to address fully water charateristics in Tien and Hau rivers. International collaboration in water management should be enhanced to solve water problems.