Provided is a complementary metal oxide semiconductor (CMOS) image sensor having a structure capable of increasing areas of photodiodes in unit pixels and expanding light receiving areas of the photodiodes. In the CMOS image sensor, transfer transistors may be formed on the photodiode, and reset transistors, source follower transistors, and selection transistors may be formed on a layer on which the transfer transistors are not formed. In such a CMOS image sensor, the areas of the photodiodes may be increased in unit pixels so that a size of the unit pixels may be reduced and sensitivity of the pixel may be improved.