Provided is a radiation detector which includes a plurality of pixels for detecting radiation, each of the plurality of pixels including a radiation absorbing layer configured to convert incident radiation photons to electric signals; a plurality of comparators configured to compare each of the electric signals with a respective plurality of reference values, in order to classify the photons in a plurality of energy bands; and a plurality of counters configured to count and store the number of photons that are classified in each of the plurality of energy bands, and which have sizes which correspond to the plurality of reference values. Accordingly, the radiation detector may increase a measurable radiation amount without a requirement that sizes of the pixels or the sub-pixels are increased.