Injection of Botulinum toxin A (BoNT-A) has been widely applied for treating various neurological diseases. It can effectively relieve focal spasticity and is highly recommended for post-stroke spasticity (PSS) according to the guidelines. When using BoNT-A for treating PSS, setting goals together with the patient before injection and continuing rehabilitation programs after injection are warranted. Most affected muscles caused by PSS are limb flexors. Only the severity of limb spasticity at modified Ashworth scale (MAS) 2 or 3 would gain benefits from BoNT-A injection. While injecting the patients, surface landmarks, electromyography, electro-stimulation, and soft tissue sonography are all helpful with targeting muscles. Adequate dosing is according to the different BoNT-A brands, the affected muscles, and the severity. Maximum dosage of single injection should be controlled for safety. After injection, patients may experience transient weakness up to about three weeks. Under the regulation of Taiwan National Health Insurance, patients may apply BoNT-A injection for treating upper limb and lower limb PSS 6 and 3 months after stroke, respectively. The details of spastic postures, related muscles, and the principles of BoNT-A usage are reviewed.