Abstract: Reasons of mechanical properties of multiple batches of Q460C steel plates produced by a steel plant lower than the requirement of GB/T 1591-2008 were analyzed through optical microscope and scanning electron microscope. The results show that the bainite structure, tiny cracks and MnS inclusions as well as Nb and Ti compound inclusions caused by center segregation were the main reasons for the unqualified mechanical properties of Q460C steel plates. And widmanstatten structure caused by improper rolling technics also lowered the mechanical properties of the Q460C steel plates.