The paper deals with the deformation and transformation behaviour of thermomechanically (TM) treated low alloyed Si-Mn TRIP steel. The aim of this work was to investigate the contribution of the factors governing the deformation and transformation process of conditioned austenite. Variation in strain and temperature parameters of TM treatment of TRIP steel samples resulted in formation of different complex microstructures. The deformation behaviour of TRIP specimens of different multiphase structures was tested in incremental neutron diffraction in situ tensile testing. It was proved that neutron diffraction technique is very convenient method for retained austenite (RA) transformation of the retained austenite with respect to monitoring of transformation quantification of retained austenite and rising internal stress in structural phases.