Tensile strength and 0.2% proof stress of T6-treated modified 6061 aluminum alloys with 0.4 and 0.8 mass% Mn instead of 0.25 mass% Cr are nearly equal to those of T6-treated non-modified 6061 aluminum alloy when water-quenching is conducted after solution treatment at 803 K for 1.8 ks after homogenization at 803 K for 36 ks. Crack initiation energy, E-Ii, obtained from Charpy impact tests on T6-treated modified 6061 aluminum alloys, where Mn between 0.4 and 0.8 mass % is substituted for 0.25 mass % Cr tends to increase slightly with increasing the content of Mn and cooling rate. While static fracture toughness, J(in), obtained from static fracture toughness tests on T6-treated modified 6061 aluminum alloys tends to be constant with increasing the content of Mn and to increase with increasing cooling rate after solution treatment. The relationship between E-Ii or J(in) and tensile strength of T6-treated modified and non-modified 6061 aluminum alloys shows a positive linear one in the range of tensile strength from 250 MPa to 350 MPa. T6-treated modified 6061 aluminum alloy, where 0.4 mass% Mn is substituted for 0.25 mass% Cr, shows the best balance of strength and toughness. It is possible to substitute Mn for Cr in non-modified 6061 aluminum alloy by judging from the point of view of the balance of strength and toughness when WQ is conducted after solution treatment at 803 K for 1.8 ks after homogenization at 803 K for 36 ks.