A novel lath-shaped to phase has been observed in the 0Crl3Ni4Mo low carbon martensitic stainless steel after quenching from high temperature, The orientation relationship is determined as: [011]alpha//[2 (11) over bar0]omega and (2 (1) over bar1)alpha//(01 (1) over bar0)omega. Using in situ heating transmission electron microscopy (TEM), we have investigated the microstructure evolution of the steel during heating. It has been found for the first time that the to phase dissolves gradually during heating. Meanwhile, another newly identified nanometer scale phase precipitates homogeneously distribute within the martensite matrix. The lambda phase precipitates have sizes of several nanometers and exhibit an orientation relationship with the matrix as follows: [011]alpha//[0001]lambda and ((2) over bar 00)alpha//(1 (2) over bar 10)lambda. The results are considered to contribute to a better understanding of the microstructure evolution of the martensitic steels during quenching and tempering. (C) 2015 Elsevier B.V. All rights reserved,