Pure organic luminogens with fluorescence-room temperature phosphorescence (RTP) dual emission have significant fundamental importance and promising applications. So far, most dual emissive materials, however, are molecular compounds; polymers still remain rare. Herein, we revisited the emission of poly(ethylene terephthalate) (PET), which is weakly luminescent in dilute solutions but becomes highly emissive in concentrated solutions and solids, demonstrating concentration-enhanced and aggregation-induced emission (AIE) characteristics. Furthermore, the efficiency of the films is enhanced with increasing crystallinity, accompanying the presence of RTP. Such crystallization enhanced dual emission is understandable taking account of conformation rigidification. Moreover, despite there is merely a single phenyl ring, emission maxima at around 400, 450, and 480 nm are observed in concentrated solutions and highly crystalline films, which cannot be simply ascribed to RTP but can well be rationalized by the clustering-triggered emission (CTE) mechanism. Namely, the clustering of terephthalate units results in efficient through-space electronic communications, thus generating extended delocalization beyond those of dimers. This study not only unveils new features of the PET emission, which may enlighten emerging applications, but also provide new insights into the construction of novel luminogens free of polycyclic aromatic hydrocarbons.