Solid-state structures of regioregulated poly(3-hexylthiophene) (P3HT) and poly(3-butylthiophene) (P3BT) were investigated using Fourier transform infrared absorption (FT-IR) spectroscopy. This study revealed that a twist glass transition of thiophene twisting for P3HT exists around 300 K (T-gp). Additionally, the influence of annealing on population of glassy crystal, crystal, and plastic crystal phases was explored. The annealed sample is dominated by plastic crystal phase, and this phase increases with annealing temperature below Tgp. The frustration against crystallization in P3HT is weaker than that in P3BT. Consequently, the plastic crystal phase is formed as a dominant structure, whereas P3BT forms both a well-defined crystal in the conventional sense and glassy crystal.