Recently, two molecular packing modes of the alkyl chain in smectic A (SmA) liquid crystal phases were revealed: normal and tilted types, which are named by the orientation relative to the layer normal. This study reveals the relationship between the packing mode and thermodynamic order of the SmA-nematic (N) phase transition. Two normal type and three tilted type mesogens were subject to thermodynamic and structural experiments. The DSC results showed that the SmA-N phase transitions of the normal and tilted types are of the second and first order, respectively. The analysis of the intensities of reflections in wide-angle X-ray diffraction related to the periodicity of the SmA layer yielded the distribution of the mass centers of molecules along the normal to the SmA layers. The resultant distribution offered a rationale for the correlation of the thermodynamic order of the SmA-N phase transition and molecular packing modes in the SmA phases based on the Meyer-Lubensky theory.