Odorous drinking water problem occurred repeatedly in one city in northern China in recent years. The uniqueness of this odor episode lay in two aspects: (1) the odorous chemicals were quite different from common odorants, such as geosmin and 2-MIB; and (2) it occurred repeatedly in different seasons during a time span of more than 2 years. The screening of odorants was the first and one of the most important steps to address this problem. The field study eliminated the possibility of external pollution and targeted on odorous algal excretion. Odorous water samples were taken at different locations. Headspace solid phase microextraction combined with gas chromatography/mass spectrometry (GC/MS) was employed to identify possible odorants. Algal species were observed under a microscope. The GC/MS results indicated that pyrazines and aldehydes, rather than geosmin and 2-MIB, were the most possible peace breakers. Other odorants, such as ketones and thiols, were also detected at trace level. The planktonic algae were detected in high populations in the reservoir. Diatom was regarded as the most possible source of the odor. Guidelines for odorants removal in water treatment process were made to help the local water company address future odor problem.