HT9, a ferritic/martensitic steel, is a candidate structural material for next-generation advanced reactors. Its microstructure is a typical tempered martensite showing a hierarchical lath-block-and-packet structure. We investigate the specimen size effect and strengthening contribution of various microstructural boundaries manifested in the compression tests of micropillars with diameters ranging from 0.5 to 17 mu m. It is observed that micropillars with diameters larger than 3 mu m show uniform deformation and plastic flow curves comparable to the bulk flow curve. Localized deformation by a few pronounced slip bands occurs in micropillars with diameters smaller than 1 mu m, and the yield strength is reduced. Careful examination of the sizes of the microstructural features and cross-sections of the micropillars shows that the block boundaries are the most effective strengthening boundaries in tempered martensitic microstructure. The bulk mechanical properties of HT9 can be evaluated from a micropillar with diameter as low as 3 mu m. (c) 2018 Elsevier B.V. All rights reserved.