A high-efficiency, high-sensitivity absorbance detection system in a lab-on-a-chip is provided. The absorbance detection system includes detection cells having an optical pathlength ten times and/or much longer than the width of a separation channel to improve detection sensitivity, lens structures for collimating light in the detection cells, and slit structures for preventing scattered light from entering detectors. The detection cells, the lens structures, and the slit structures of the absorbance detection system are fabricated and integrated in a lab-on-a-chip. The absorbance detection system exhibits excellent absorption efficiency, detection limit, and linearity, compared to existing absorbance detection systems, and can be applied for the detection of a variety of samples. The absorbance detection system does not need labeling of the samples which saves time and costs. The absorbance detection system can be used effectively in detecting trace compounds with a high sensitivity. The absorbance detection system in a lab-on-a-chip can be used with wider applications to a variety of samples in diverse research fields, such as the drug screening field dealing with simultaneous synthesis and identification of a number of compounds based on combinatorial chemistry, the life sciences field handling trace bioactive materials such as enzymes, proteins, and amino acids, and the environmental research field which needs rapid field monitoring of contaminants.