[en] We have applied various countermeasures for worker dose reduction to plants in Onagawa Nuclear Power Station. 'Ni/Fe ratio control' has been applied to Unit 1. Due to the change of fuel cladding surface treatment, effect of Co-60 concentration reduction in the reactor water by Ni/Fe ratio control decreased. Considering such situation, 'ultra-low-crud high-nickel control' has been adopted to Unit 2. Main objective of ultra-low-crud high-nickel control is to suppress deposition of Co-60 onto primary circuit piping. Because we have obtained successful results in dose rate reduction in Onagawa unit 2, we decided to adopt this water chemistry control also in unit 3. In this paper, we describe water chemistry data under ultra-low-crud high-nickel control in unit 2 and unit 3. (author)