Optical heterodyne magnetic rotation enhanced concentration modulation spectroscopy which is a high sensitive spectroscopic technique for detection of transient and excited molecules, is employed to directly observe the rovibronic absorption spectrum of CO (d3Δ←a3Π) in the regions of 16400～16500 cm-1 and 17450～17750 cm-1 respectively. The (4,0) and (5,0) bands of CO triplet system were observed and analyzed by taking account of the perturbation interaction between the d3Δ1 (v=4),d3Δ2 (v=4) states and A1Π(v=0),D1Δ (v=0) states, respectively, d3Δ1 (v=5) and A1Π (v=1) states as well. A set of precise rotational molecular constants for the d3Δ (v=4,5) have been determined.