H13 die steels with varied yttrium (Y) content were prepared by vacuum induction melting, multiple forging, annealing and quenching treatment with stepwise heating. The effects of Y on the microstructures, internal fraction and martensitic transformation of H13 die steel were investigated using electron backscattering diffraction, transmission electron microscopy and a multifunctional internal friction meter. The results showed that the martensite start temperature first decreased but then increased with the increasing Y content, reaching a minimum in the 0.013Y-H13 steel. The refinement of the prior austenite grain size afforded more nucleation sites in Y-modified H13 die steels. The Snoek-Koster-Ke peak indicated that the solid solution of Y atoms provided additional martensitic transformation dynamics to increase the martensitic transformation rate and promote the formation of V1-V2 (sigma 3) variants during the initial stage of transformation. The transformation rate decreased in Y-modified H13 steels during the late stage of transformation (70% completed). Therefore, the addition of Y elements was beneficial for refining the size of the martensite and promoted the formation of twin-type martensite in H13 steel.