The purpose of this study was to demonstrate two cases of vertical diplopia after cataract surgery and to discuss the mechanism and types of strabismus caused by the local anesthesia with retrobulbar injection. Two cases of vertical diplopia after cataract surgeries were reported. They were operated by the same surgeon and both happened to the left eye and both presented with marked left hypotropia. The clinical findings consisted of overacting left inferior rectus (LIR) in case 1 and mixed overacting and restrictive LIR in case 2. The diplopia and left hypotropia were eliminated after muscle operation. Retrobulbar injection is a basic technique in ophthalmic practice. Although it is generally safe, complications do happen in certain cases including diplopia and strabismus. Ophthalmologists should know about the orbital anatomy and learn to prevent any accidental insult to extraocular muscles.