By using fluorescence spectroscopy together with d.s.c. and WAXD, structure analysis for main-chain thermotropic LC polyesters (PB-n), containing n = 7-18 methylene units, was carried out. The strong excitation-wavelength dependence of fluorescence and red shift of fluorescence peak wavelengths excited at various wavelengths during heating for all PB-n were found to depend on different degrees of overlap between mesogenic biphenyl moieties. The red shifts for fluorescence excited at 320 nm from room temperature to crystalline-S-H transition temperature for PB-16 (26 nm) and PB-18 (36 nm) are larger than those for PB-8 (14 nm) and PB-10 (16 nm), owing to the mobile packing structure in crystalline phase for PB-16 and PB-18 with flexible and longer spacer lengths. According to the much longer average fluorescence peak wavelengths for PB-16 (399 nm) and PB-18 (401 nm) in S-H phase than those for PB-8 (381 nm) and PB-10 (385 nm), the degree of overlap between two mesogenic biphenyl moieties in SH phase for PB-n was proved to decrease and the interaction between a carbonyl group and a phenyl ring increased with its increasing methylene units. Fluorescence spectroscopy was demonstrated to be effective in proving the difference in molecular interaction between mesogenic moieties for LCP with various spacer lengths.