Aluminum with a high B4C content are valuable as a neutron absorber for spent nuclear fule pool storage rack,welding of B4C/6061Al using conventional fusion methods degrades the joint properties due to solidification induced structure and chemical reactions. B4C/6061Al neutron absorbers were fabricated by powder metallurgy with the B4C volume fractions of 0 ~ 30%. Plates of 4 mm thickness and 30% B4C/6061Al neutron absorbers was prepared and successfully butt joined using friction stir welding (FSW). The weld displays the smooth appearance. The micro-mechanical properties in welded joints of FSW weld nugget (WZ), Thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ) and base metal (BM) were investigated by nanoindentation method, and the tensile tests were carried out.Furthermore,the microstructure of indentation and fracture surface were characterized by scanning electron microscope. The results showed that in the same region, with the distance(d＜11 μm) of the interface between particle and matrix increased, the micro-area of hardness and elastic modulus decreased overall. After the micro-area equalization, different regions of hardness and elastic modulus from high to low were WZ, TMAZ, BM and HAZ. The pressure into the power recovery rate in WZ, TMAZ, HAZ and BM were 28.10%, 25.14%, 31.76% and 29.30%. The performance difference in the welding joint of different areas was caused by the different degree of plastic deformation and the thermal cycle in different area. A joint efficiency of 85.7% was realized under the experimental conditions, and the welded joint failed at HAZ.