Given an image and text description, visual grounding will find target region in the image explained by the text.It has two task settings: referring expression comprehension (REC) to estimate bounding-box and referring expression segmentation (RES) to predict segmentation mask. Currently the most promising visual grounding approaches are to learn REC and RES jointly by giving rich ground truth of both bounding-box and segmentation mask of the target object. However, we argue that a very simple but strong constraint has been overlooked by the existing approaches: given an image and a text description, REC and RES refer to the same object. We propose \textbf{Lo}cation \textbf{A}ware \textbf{Trans}former(LoA-Trans) making this constraint explicit by a {\it center prompt}, where the system first predicts the center of the target object by simple regression, and feeds it as a common prompt to both REC and RES. In this way, the system constrains that REC and RES refer to the same object. To mitigate possible inaccuracies in center estimation, we introduce a query selection mechanism. Instead of random initialization queries for bounding-box and segmentation mask decoding, the query selection mechanism generates possible object locations other than the estimated center and use them as location-aware queries as a remedy for possible inaccurate center estimation. We also introduce a TaskSyn Network in the decoder to better coordination between REC and RES. Our method achieved state-of-the-art performance on three commonly used datasets: Refcoco, Refcoco+, and Refcocog. Extensive ablation studies demonstrated the validity of each of the proposed components. We will release the code later.