An online measurement method of tree canopy volume using vehicle-borne 2-D laser scanning was proposed. Firstly, the point cloud data of trees was obtained by using a vehicle-borne 2-D laser scanner. After coordinate transformation, trees within the region of interest were detected. Then trunks were recognized by using their vertical distribution characteristic and trunk distances were obtained. In the case of canopy continuity, tree segmentation was performed by finding minimum data frame between trunks. Crown thicknesses were computed by subtracting the crown outer edge distances from the corresponding trunk distance. Finally, tree canopy was discretized into small boxes and its volume was computed by using crown thickness, distance between adjacent measuring points in the vertical direction, vehicle speed and scanning circle. To meet the demand for online measurement, data was saved in FIFO buffer which included all information required for the calculation of tree canopy volume. New data frame was written to the end of the buffer and the processed data frame was read from the beginning of the buffer. The experimental results showed that in the continuous and discontinuous canopy scenarios, trees and trunks were detected effectively and tree canopy volume was measured online accurately. In the experiment, the average processing time per frame was 2.0 ms and the maximum processing time was 15.6 ms, which was less than laser scanning period (25ms) and met the needs of real-time processing.