Phase behavior of undecane-dodecane mixtures (n-C11H24-C12H26, C11-C12) in bulk and confined in SBA-15 (pore diameters 3.8, 7.8, and 17.2 nm) has been studied by differential scanning calorimetry. Phase diagram of the confined C11-C12 system shows dependence on pore size, indicating simple behavior compared with the bulk. In the three systems, melting points of the confined C11, C12 and the mixtures (xC12 = 0.1–0.9) present a linear relation with the pore diameter from 3.8 to 17.2 nm. Melting behavior of the confined C11-C12 mixtures is closely as the confined C11, C12. Melting temperatures of the three systems have been fitted as a function of mole fraction xC12.