Highly selective tunable receiver has been studied experimentally for optical packet dense WDM systems. Performance evaluation of the receiver using new test methods confirms the feasibility of ultra-dense WDM spaced by several GHz. The receiver consists of a fiber loop which contains fiber delay line, tunable optical band-pass filters, an erbium-doped fiber amplifier and an optical switch, an optical switch at the receiver input port, and a photodiode followed by a switch at the fiber loop oputput port. Target packets are selected from input optical packet WDM signals by time and wavelength windows realized by the optical switches, and the tunable filters, respectively. Wavelength selectively enhancement is attained by the target packet multiple circulation in the loop. Optical packet ultra dense WDM test signal was generated by multiplexing 8 optical packets, where channel spacing ranged as narrow as from 0.62GHz to 3.62GHz in 500MHz step. The receiver fiber loop employed tunable disk filters, a gain stabilized EDFA, low PDL isolators and a magneto-opticla switch to realize high selectivity, high stability and low noise. The receiver has demultiplexed the target optical packets from optical packet ultra-dense WDM spaced by 1.1GHz, attaining high SNR and stable operation.