The thermal expansion of tetrakis (alkylthio) tetrathiafulvalenes (abbreviated to TTCn-TTFs. (n = 1-11)) having alkyl chains with n carbon 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 delta(l)(= Delta l/Delta T) is suggested for correlating intermolecular interaction of TTCn-TTF crystals. For the compounds with larger n, n greater than or equal to 8, the linear thermal expansion along the c-axis, delta(c), is smallest, while delta(b) is larger and delta(a) intermediate. This is consistent with the intermolecular interaction analyses for the crystal structure of TTCn-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.