Abstract: Partial volume effect(PVE) impairs image quality and quantitative accuracy. PVE in dual-head coincidence detection(DHC) images was investigated. Eighteen emission scans were performed with total radioactivity 15.15～204.33 MBq (radioactivity concentration 4.33～58.38 kBq/mL) for a cylindrical phantom (Toshiba AZ-618 ECT phantom) using DHC system (GE. Millennium VG Hawkeye). The images were reconstructed using OSEM algorithm with post filter Hanning (cutoff frequency 0.6). The results show that cold lesions' recovery coefficient increases with increasing background's radioactivity concentration and decreases with increasing cold lesions' size. It suggests that PVE of cold lesion in DHC is related to radioactivity concentration as well as the size of the lesion, and its severity increaseswith increasing radioactivity of the background.