All-atom molecular dynamics (MD) simulations were used to investigate the thermal phase behavior of two hydrated phospholipids, namely, DPPC and DPPE, at the atomic level. The trajectories in the MD simulations clearly identified the structures of DPPC in the crystalline (L-c), gel (L-beta), ripple (P-beta), and liquid-crystalline (L-alpha) phases and those of DPPE in the Lc and La phases. The physicochemical and structural properties of these phases agree well with the experimental results. Moreover, the structural transformations between phases were observed. In the L beta phase, forces are directed in opposite directions in the upper and lower layers of the bilayer. These forces, which are due to the thermal motion of each monolayer, strongly influence the series of phase transitions from L beta to P beta. The MD simulations in this work can provide an understanding of the dynamics of the lipid bilayer in each thermal phase and suggest the mechanism that generates the P beta phase.