Abstract: Plant growth relies on the conversion of carbon dioxide in air to carbohydrates. Naturally，one expects that the plant growth rate depends on carbon dioxide concentration in air. Indoor air quality is of paramount importance to human health. Motivated by the possibility of monitoring indoor carbon dioxide level with potted plants，we developed a sensitive optical tracking method with a webcam to measure the growth rate(height increase in the stem)of soybean(Glycine max L．)in real time over a broad carbon dioxide concentration range，from 400 ppm to 1700 ppm. An imaging processing algorithm was developed to track the growth rate from captured images. A paper label was stuck to the stem of plant. The algorithm first plotted the image intensity profile along the hypocotyl，identified a position where the intensity profile of the marker had the largest gradient(e.g.，the edge of the paper label)，and determined the vertical shift in the position(growth)from the time sequence images. Result shows the growth rate increases with carbon dioxide concentration from 6 μm?min－1 at 400 ppm to 22 μm?min－1 around 800 ppm，but decreases when the concentration exceeds 900 ppm. Our optical tracking method has the following advantages：(1)plants are cheap and available everywhere，(2)a camera can achieve real－time measurement without additional equipment，(3)this method produces no pollution and chemical reagents are not necessary.