Objective　To provide morphological data of an algorithm for optimal the site and needle length for gluteal intramuscular injection in adults.　Methods　(1) Twenty-three embalmed cadavers were injected with red latex. The buttock injection sites were identified as a probe. Dissection, measurement and photography were used to document the precise soft tissue thickness and relationship between the sciatic nerve and the gluteal intramascular injection sites in the gluteal region; (2) Six fresh cadavers underwent whole-body lead oxide injection for three-dimensional reconstruction using a spiral CT scanner and specialized volume-rendering software.    Results    Mean soft tissues thickness at injection site was (6.63±1.06) cm, the depth from intergluteal fissure to iliac periosteum was (4.37±0.62) cm, the depth from skin to intergluteal fissure was (2.26±0.89) cm, the length from injection needle to main trunk of superior gluteal vessels and sciatic nerve was (5.87±0.93) cm and (7.46±0.94)cm respectively in trichotomy way. The length between reference point and sciatic nerve was (3.63±0.68) cm in crossing way.    Conclusions    The finding that buttock subcutaneous fat thickness exceeds the length of the most commonly used needle for intramuscular buttock injection (2.7~4.0cm), suggests that the currently used injection technique may not be therapeutically optimal. We suggest that a needle longer than 6 cm should be used in for intramuscular buttock injections in adults.