The Strengthening behavior by the repeated strain againg and the low temperature aging process in maraging steels containing 18%Ni, 8%Co and 5%Mo have been investigated by specific heat and electric resistance measurements and also by the tensile test. The effect of cold working in the solution treated condition on the tensile strength and on the low temperature aging reaction is not significant. On the specimen aged at 475°C for a few minutes, cold working, however, greatly increases the tensile strength and also promotes the low temperature aging reaction in the following aging. During the first aging, Mo rich zones are formed on dislocation lines and pin down them, and the subsequent cold working increases the dislocation density by multiplication. The newly introduced dislocations supply sites for Mo zone formation, resulting the promotion for the reaction of low temperature aging in the following aging. Therefore, the repetition of aging and cold working results in a remarkable increase of the tensile strength. For example, the tensile strength of specimens treated 10 cycles repeated strain aging (5% reduction in area and 475°C-2 min aging) reaches 1700 MPa, which exceeds the highest tensile strength 1530 MPa attained by the static aging at 475°C for 20 h.