Biomembranes of cells are mainly composed of phospholipid bilayers, including various kinds of small organic molecules. To understand widely the effects of these small molecules on a phospholipid bilayer, systematization of the effects is desired with focusing on the molecular structures of the additive molecules. Cholesterol is well known to disturb the molecular ordering of a lipid bilayer in the gel (L_β' and P_β') phases, resulting in softening of the bilayer and lowering of the transition temperature between P_β' and L_α' phases. On the other hand, in the present study, we found n-alkane makes the molecular arrangement more ordered in the L_β' phase, resulting in rigidification of the bilayer and raise of the transition temperature. Bulky core-like molecules such as cholesterol and the molecules with a linear and flexible chain contrastingly affect the bilayers due to the contrary effects on the ordering of acyl chains in the bilayer. Indeed, cis-stilbene, the bulkier molecule, disturbs the acyl chains more largely than the trans-isomer, lowering the transition temperature more drastically. When a core part and an alkyl chain coexist in a molecule like nCB, the effects of these parts compete each other. The effect of a whole molecule changes depending on the length of an alkyl chain.