Abstract Polyacenic semiconductor (PAS) is obtained by pyrolyzing phenol-formaldehyde resin (PFR). The properties of PFR heat-treated at different temperatures are investigated. The lithium intercalation capacity of PAS as a function of heat-treatment temperature (HTT) exhibits a maximum at around 700 °C. A knee appears at 700 °C, not only in the plot of atomic ratio [H]/[C] versus HTT, but also in the plot of conductivity versus HTT. For PAS with a HTT of 700 °C, the maximum in the ratio of the relative intensity of Raman spectra at 1360 cm −1 corresponds to nanometer graphite, and that at 1580 cm −1 to graphite. A reasonable explanation of these phenomena is the transformation of nanometer graphite to graphite.