The solubility of trehalose in the water + methanol solvent system was measured with the mole fraction of water ranging from 0.000 to 0.700, at temperatures from (293.15 to 313.15) K, using the gravimetric method. Two kinds of crystals were collected and measured by differential scanning calorimetry and X-ray diffraction to prove the change of crystal habit for dihydrate trehalose from granular to powderlike at low water content. The turning points of every solubility curve were the critical points of crystal habit transition from the granular dihydrate trehalose (higher than the critical points) to the powderlike dihydrate trehalose (lower than the critical points). The mole fraction of the critical water content ranged from 0.160 to 0.250. The combination version of the Jouyban-Acree and vant Hoff models was used to separately correlate the solubility data lower and higher than the critical points by nonlinear surface fit. The root-mean-square deviation (rmsd) values for the powderlike and the granular dihydrate trehalose solubility data were 1.4900.10(-4) and 3.5919.10(-4), respectively, which shows the model correlated the data well.