In the present study, the microstructure, toughness and cracking behavior of a batch of X12CrMoWNbVN10-1-1 martensitic heat resistant steels used for manufacturing steam turbine blades in power plant were investigated. The manufacturing route of the X12CrMoWNbVN10-1-1 martensitic steels was casting, forging and heat treatment. These martensitic steels were taken from different positions in the same ingot, but they exhibited different toughness, which were marked as two groups: (i) the matrix located in the core of the ingot, hereinafter referred to sample A; (ii) the matrix located in the edge of the ingot, hereinafter referred to sample B. Impact toughness was characterized by Instrumented Charpy impact test, fracture toughness was determined by single edge notch tension (SENT) test in terms of double-clip gauge method, and crack propagation behavior was observed by in-situ single edge notch bend (SENB) test. Optical microscopy (OM), scanning electron microscopy (SEM) and high-resolution energy dispersive X-ray spectrometry (EDX) were used to examine the microstructure and cracking features. The impact toughness of sample A was much lower than that of sample B, which was proved to be attributed to the large amount of NbC precipitation in sample A. The clustered NbC particles could induce brittle cracking and thus significantly reduced absorbed energy for stable crack propagation, resulting in the poor impact toughness of sample A. It was believed that the large amount of NbC in sample A was precipitated directly from liquid metal during casting process, which might be related to the high niobium content of the core of the ingot due to the low cooling rate. The results revealed that the large number of clustered NbC particles formed in the casting process not only cannot promote precipitation-induced reinforcement, instead might be detrimental to toughness.