Novel polyphenylene oxide/polyamide 6 (PPO/PA6) blends were synthesized via in situ polymerization of epsilon-caprolactam with PPO dissolved in it. The introduction of 10 wt% PPO into PPO/PA6 led to phase inversion of the blends, which was nearly completed by incorporating 15 wt% PPO into the blends. A single crystallization temperature (T (c)) of PA6 was detected for PPO/PA6 with 1-4 wt% PPO, while double T (c) existed in the blends with 6-15 wt% PPO. After eliminating previous thermal history, PPO/PA6 containing no more than 6 wt% PPO gave a single melting point (T (m)), but the blends with 10-15 wt% PPO exhibited double T (m). Increasing PPO content in PA6 resulted in the transformation of its crystal form from alpha-crystal to gamma-crystal, which might be attributed to hindrance of crystallization of PA6 particles in PPO-rich phase.