Comparative insecticidal efficiency between compounded pyrethroids and its preparations cooperated with several synergists was studied. As main components, Group A contained Permethrine Phthalthrine, Group B contained Permethrine Furameth rine and Group C contained Phthalthrine Dichlorvos. As synergists Piperonyl Butoxide (P.B.O.), N-Octylbicycloheptene dicarboximide (MGK-264), Octachlorodipropylether (S-421) was cooperated in each group. These preparations was treated to either Blattella G. or Musca D. for comparing about insecticidal efficiency[Knock-down rate (KT_(50), KT_(90)) and cumulative mortaility(percent/hrs)]. When the synergists was added to each Group, the insecticidal efficiency was improved. The most potent synergists was P.B.O. and the potency was decreased in the rank order of S-421$gt;MGK-264. The more the concentration of synergists was increased, the more the insecticidal effciency was improved. The rank order was 1:5≥1:4$gt; 1:3 in all three groups. The mortality (percent) according to time was similar to either KT_(50), or KT_(90-). Knock-down rate was appropriate parameter for the indicator about potency.