An apparatus for measuring a blood component using the trans-reflectance of light irradiated to a transflective member disposed behind a test site. The apparatus includes an optical source unit irradiating light to the test site of a test subject; a transflective unit transflecting light transmitted through the test site; a detection unit detecting the transflected light; and an analysis unit analyzing the transflected light detected in the detection unit. A related method includes irradiating the light to the test site; transflecting light through the test site, detecting the transflected light, and analyzing the detected light. Blood component measurement can be easily performed without collecting blood by analyzing the transflected light with a high optical power containing much information about a blood component.