The effects of benzene addition on PAHs formation are studied in a stabilized laminar low-temperature flame under an atmospheric pressure, to understand the processes of soot formation in the internal combustion engines. The burner was operated with a diethyl-ether/air mixture at an equivalence ratio of 3.0, containing 1.6-ppm benzene. Single-ring aromatics and PAH ranging from two- to four-fused aromatic rings were identified by microprobe sampling and a liquid chromatograph with a fluorescence detector. Both the cool- and blue-flame appearance were advanced with the benzene addition. The benzene addition suppressed the growth of PAH, contrary to the more production of single-ring aromatic hydrocarbons, such as, phenylacetylene and styrene. The phenol formation was detected at the time when a cool flame appeared. It is presumed that the benzene-origin HO_2 and OH formed during the low-temperature oxidation have enhanced the cool-flame reaction, even if the amount of benzene addition was not high.