The transition of crystal growth is revealed in ultrathin poly(ethylene oxide)/poly(methyl methacrylate) (PEO/PMMA) blend films (film thickness D < 3 nm) as the crystallization condition changes from vacuum to humidity. The PEO crystallizes into a fractal-like branched structure in a vacuum from a state mixed with PMMA, but in humidity into one reminiscent of dense branching morphology (DBM). The crystal lamellae in both conditions orient flat-on, but the DBM-like structure is much thicker and wider, indicating a crystallization more close to equilibrium. Results show that as a PEO/PMMA blend film is exposed to humidity, PEO chains demix with PMMA and segregate on a PMMA layer before or during crystallization. The crystal growth transition is attributed to the change in the factors that dominate the crystal growth at crystal growth fronts, that is, the kinetic and surface tension effects, as a result of changing the polymer states in the blend films. The crystal growth of the demixed PEO on the PMMA layer is dominated by a strong surface tension effect, while crystal growth of PEO in a blend/mixed state is dominated by either a kinetic effect or a weak surface tension effect depending on the film composition. The compositional dependence of the crystal growth in humidity is also elucidated on the basis of the changing of polymer states in the films.