The odd–even effect of the alkyl chain length of rubbed polyimide Langmuir–Blodgett (LB) films on the extrapolation length of 5CB has been successfully evaluated by measuring polar anchoring strength. The extrapolation length of 5CB for rubbed PI‐LB films with even numbers is small compared with odd numbers for alkyl chain lengths of greater than 7 carbons. The extrapolation length of 5CB on rubbed PI‐LB films with odd numbers increases gradually as the temperature increases but tends to diverge near the clearing temperature (Tc=35.3 °C). The extrapolation length diverges because of rapidly decreasing surface order nearTc. It is suggested that the polar anchoring strength on rubbed PI‐LB films with an even number is strong because of relatively high surface ordering caused by more crystalline surfaces. Finally, it is concluded that the odd–even effects of the polar anchoring strength in NLCs are strongly related to the character of the polymer and observed clearly for long alkyl chain lengths. ©1995 American Institute of Physics.