Domain adaptation has been extensively explored in object detection. Through the utilization of self-training and the decoupling of adversarial feature learning from the training of the detector, current methods make detectors more transferable and ensure their discriminability. However, the presence of low-quality pseudo labels during self-training introduces noises to the training phase and thus degrades the model performance. To tackle this challenge, we introduce an I-adapt framework, whose IoU Adapter accurately predicts the Intersection over Union (IoU) between predicted boxes and their corresponding ground-truth boxes in both source and target domains. This enables an effective measure for the pseudo-label quality. Based on this measure, we propose a re-weighting strategy, which enforces the detector to focus on learning from high-quality pseudo labels. We achieve state-of-the-art (SOTA) performance in several cross-domain object detection tasks, proving the effectiveness of I-adapt.