The microstructure and strength of the lath martensite in 15-5 PH stainless steel is analysed as a function of tempering temperature. Based on comprehensive microstructure characterisation, it is found that with increasing tempering temperature, the size of the prior austenite grains, martensite packets and block width increases. In contrast, the lath width remains almost unaffected by the tempering temperature. The yield strength of the low-carbon steel is further related to the packet size and block width. This suggests that the martensite block width is the most effective parameter when analysing the strength-structure relationship of lath martensite. To facilitate the experimental interpretation, high-resolution crystal plasticity finite element modelling is also integrated.