Liquid crystal (LC) blue phases (BPs) are mesoscopic three-dimensional “soft crystals” with unit cell sizes orders of magnitude larger than those of classic, atomic crystals. The directed self-assembly of BP soft crystals on chemically-patterned surfaces offers us the unique opportunity to make detailed observations of martensitic phase transformations in the soft material systems. Thereby, we enrich our understanding of such diffusionless phase transformations in both soft and hard matter, including their similarities and differences. Herein, by in-situ soft X-ray scattering, we carry out an in-depth, quantitative study of a soft crystal martensitic transformation, and therefore, the collective motion of LC molecules which enables the local reorganization of the three-dimensional LC mesostructure. For the first time, the formation of twin lamellae is clearly identified during phase transformation from BPII to BPI, further corroborating the martensitic nature of this transformation.