Recently, machine learning techniques, especially supervised learning techniques, have been adopted in the Intrusion Detection System (IDS). Due to the limit of supervised learning, most state-of-the-art IDSs do not perform well on unknown attacks and incur high computational overhead in the Internet of Things (IoT). To overcome these challenges, we propose a novel IDS based on unsupervised techniques, namely, UTEN-IDS. UTEN-IDS uses the ensemble of autoencoders to handle the network data and performs the anomaly detection by an Isolation Forest algorithm. The effectiveness of the proposed method is verified using two benchmark datasets. The results show that our approach has significant advantages in classification performance and proves its utility in the IoT network when compared to other approaches.