Many of alkylureas (H 2 NCONHC n H 2n+1 , C n –U) exhibited phase transitions and those found for C 4,8 –U had fairly large entropy of transition, indicating formation of quasi-plastic phases. X-ray structure determinations of C 2,5–14 –U revealed a characteristic common hydrogen bond network to result in the formation of two-dimensional, plate-like supramolecules having crossed arrangement of alkyl groups. The crystal structures of C 2,5–14 –U were found to be built by stacking the corresponding supramolecules along the a axis. The phase transitions of C 10,13,14 –U and those of C 8,12 –U were found to accompany shifts of supramolecules along the crystal c axis and along both the b and c axes, respectively. The transitions found for C 4,8 –U were proved to be of order–disorder type associated with the disordering of the alkyl groups. The transition of C 4 –U was found to be associated with a drastic twist of the plane of the NCON moiety. The transition temperatures of C 4 –U and C 8 –U were depressed significantly by doping C 3 –U and C 7 –U, respectively, to form corresponding mixed crystals.