The thermal expansion of tetrakis (alkylthio) tetrathiafulvalenes (abbreviated to TTCn-TTFs. (n' 1–11)) having alkyl chains withncarbon atoms has been investigated from room temperature down to about 100 K by means of the four-circle X-ray diffraction technique. A linear thermal expansion parameter δl(' δl/δT) is suggested for correlating intermolecular interaction of TTCn-TTF crystals. For the compounds with largern,n⩾ 8, the linear thermal expansion along thec-axis, δc, is smallest, while δb, is larger and δaintermediate. This is consistent with the intermolecular interaction analyses for the crystal structure of TTCa-TTF compounds. The molecular fastener effect is also found in the thermal expansion measurements. The contributions of the molecular fastener effect and the packing effect of the end C-atoms in the alkyl chains of TTCn-TTF (n'8–11) to the thermal expansion are discussed.