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We have tried to confirm the sign inversion of the photocarrier in regiorandom poly(3-octadecylthiophene), PAT18, by means of the time-of-flight (TOF) method. It was found that the hole mobility decreases with increasing temperature. At the temperature near the solid-liquid phase transition, the hole mobility decreases drastically, and eventually the melting temperature, it was impossible to evaluate the hole mobility from noisy transient photocurrent. On the other hand, transient photocurrents based on electron transport are observed above the melting point. The negative carrier mobility was evaluated in the range from 10-6 to 10-5 cm2/Vs, which is comparably the hole mobility at the solid state. Moreover, the electrical conductivity changes drastically at the instant of the solid-liquid phase transition, and transport in the liquid state was not influenced remarkably by applied hydrostatic pressure, which suggests electronic transport and not ionic transport. These facts suggest that the same mechanism, for example interchain hopping limits carrier transport, and the negative carrier is electron. This unique phenomenon is interpreted as modulation of electronic energy state caused by conformational change of the main chain.