The eight members of the prostanoid receptor family belong to the class A G protein-coupled receptors. We investigated the evolutionary relationship of the eight members by a molecular phylogenetic analysis and found that prostaglandin E-2 receptor subtype 2 (EP2) and prostaglandin D-2 receptor (DP) were closely related. The structures of the ligands for the two receptors are similar to each other but are distinguished by the exchanged locations of the carbonyl oxygen and the hydroxy group in the cyclopentane ring. The ligand recognition mechanisms of the receptors were examined by an integrated approach using several computational methods, such as amino acid sequence comparison, homology modeling, docking simulation, and molecular dynamics simulation. The results revealed the similar location of the ligand between the two receptors. The common carboxy group of the ligands interacts with the Arg residue on the seventh transmembrane (TM) helix, which is invariant among the prostanoid receptors. EP2 uses a Ser on TM1 to recognize the carbonyl oxygen in the cyclopentane ring of the ligand. The Ser is specifically conserved within EP2. On the other hand, DP uses a Lys on TM2 to recognize the hydroxy group of the omega chain of the ligand. The Lys is also specifically conserved within DP. The interaction network between the D(E)RY motif and TM6 was found in EP2. However, DP lacked this network, due to the mutation in the D(E)RY motif. Based on these observations and the previously published mutational studies on the motif, the possibility of another activation mechanism that does not involve the interaction between the D(E)RY motif and TM6 will be discussed.