There is an asymmetric carbon atom in lactic acid molecule, which has two different spatial configurations in the way of spatial arrangement, and also called enantiomer. The two spatial configurations are both in a relation of material object to mirror image, equivalent to the relation between right hand and left hand and being a chiral molecule. Generally, the substance with choral molecule features a special physical property—optical rotation. That is to say, it will enable the vibration plane of polarized light to rotate at a certain angle when detected by polarimeter. Also, the optical rototary substance is called optically active substance. So, the lactic acid has optical activity.