Various kinds of solid state organic lasers have been reported using dye-doped polymeric waveguides and conjugate polymer materials from visible to infrared wavelength region both in thin film and bulk structures. Although there are a few reports concerning the photostability of the organic lasers, most of the lasers are suffering from a degradation of the laser output in the continued shots of pump lasers less than ∼104 [1,2]. On the other hand, an organic crystal of thiophene/phenylene co-oligomer (TCPO), consisting of in-line thiophene and benzene rings, have shown peculiar optical properties such as strongly polarized lasing emission. 1,4-Bis(5-phenylthiophen-2-yl)benzene (AC5) is one of the TCPOs' family and has been researched much for optoelectronic applications [3]. In this study, a highly photostable lasing characteristic of the AC5 organic crystal will be reported in the durability test.