A series of thermotropic liquid crystal poly (4, 4' -oxyazoxybenzene ester) (PAB) with different number of methylene group in flexible unit has been synthesized. The odd-even effect of phase transition temperature has been obviously observed by DSC technique. The nature and the main factor of odd-even effect have been investigated by dynamic FTIR. The effect of symmetric structure of mesogenic unit in main chain on the phase transition temperature of liquid crystal polymer has been discussed by comparing with poly (2, 2' -dimethyl, 4, 4' -oxyazoxybenzene ester) series.