Internet of things (IoT) is used to provide real-time data collection and analysis of the target area by the cooperation of low-cost devices. The authentication towards multiple devices has become the research hot-spot considering of the requirement in real applications. Sensitivity and privacy of data have caused widespread concerns because low-cost devices are neither tamper-proof nor capable of performing public key cryptography efficiently. However, many researchers only focus on the authentication between two devices in the network. They ignore the authentication among group devices attached to one network. In this paper, we propose A Practical Group Authentication Scheme for Smart Devices in IoT. Note that one device group to be authenticated consists of a group of smart devices. The personal digital assistant (PDA) as the group leader controls authentication operations in its group. From the security analysis, our scheme can resist to various attacks. In addition, the performance analysis shows that our scheme has lower computational cost than the existing scheme.