BLCP Power plant Unit 1 and Unit 2 in Thailand are sub-critical pressure drum type steam power plants. Both the Units changed their water treatment method from the conventional AVT(R) (All Volatile Treatment (Reducing)) and PT (Phosphate Treatment) to the latest environmentally-friendly High-AVT(LO)(All Volatile Treatment (Low Oxygen)), which is free from hydrazine and sodium phosphate. This paper presents this change, which resulted in good operational achievements.