Nowadays, Light Detection And Ranging (LiDAR) has been widely used in autonomous vehicles for perception and localization. However, the cost of a high-resolution LiDAR is still prohibitively expensive, while its low-resolution counterpart is much more affordable. Therefore, using low-resolution LiDAR for autonomous driving perception tasks instead of high-resolution LiDAR is an economically feasible solution. In this paper, we propose a novel framework for 3D object detection in Bird-Eye View (BEV) using a low-resolution LiDAR and a monocular camera. Taking the low-resolution LiDAR point cloud and the monocular image as input, our depth completion network is able to produce dense point cloud that is subsequently processed by a voxel-based network for 3D object detection. Evaluated with KITTI dataset, the experimental results shows that the proposed approach performs significantly better than directly applying the 16-line LiDAR point cloud for object detection. For both easy and moderate cases, our detection results are comparable to those from 64-line high-resolution LiDAR. The network architecture and performance evaluations are analyzed in detail.