A series of urethane-substituted polydiacetylenes (PDA-mUEt) with different alkyl chain lengths (m = 4, 5, and 6) in side chain substituents of (CH2)(m)OCONHC2H5 were synthesized. The effect of the alkyl chain length on the thermochromic phase transition was investigated by measurements of reflectance spectra for the PDA-mUEt crystals. The thermochromic phase transition shows an even-odd effect for the alkyl chain length. The PDA-5UEt crystal undergoes a reversible thermochromic phase transition between A (blue) and B (red) phases, while the even-numbered PDA-mUEt crystal turns to the B phase on heating but does not revert completely to the initial A phase even when cooled down to room temperature. The difference in the thermochromic phase transitions of PDA-mUEt crystals with varying alkyl chain length was also studied by the differential scanning calorimetry (DSC).