A new system that uses the total reactivity with ozone (RO3) was developed for measuring biogenic volatile organic compounds (BVOCs) emitted from vegetation into the atmosphere. The decrease in ozone caused by the reaction with VOCs was monitored at the ppbv level by dual chemiluminescence detectors (CLDs) based on the NO-O3 reaction. Ozone was monitored by the dual CLDs before and after the reactor to correct for fluctuations in the ozone concentration. Such dual detectors can remove the interference caused by water vapor. A glass double-tube was adopted as the reactor. The loss rate of ozone to the wall was typically (6 ± 3) × 10–4 s–1. Gaseous cyclohexane was also added to the sample before it was introduced into the reactor to scavenge secondary OH radicals. From the characterization of the RO3 analyzer using the standard VOC sample, the dependence of ozone reduction on the reaction time and reactivity were shown to agree with theoretical predictions. A calibration procedure for determining the reaction time was also established. Consequently, the detection limit for RO3 in a 57-s reaction was determined to be 1.4 × 10–4 s–1 (S/N = 3), which corresponded to 27 ppbv of limonene. It was confirmed that the RO3 analyzer was capable of measuring BVOC levels. Finally, a practical trial was conducted in which BVOCs emitted from a real needle-leaf tree were monitored. BVOC emissions from the tree were detected and a significant increase in RO3 was observed when the tree was irradiated with light.