Probe molecule technique has been applied to clarify the reaction pathway for selective synthesis of isoalkanes in the gasoline range from syngas catalyzed by zeolite Y supported Ru-Pt bimetallic catalysts. Reaction of 1-octene over the metal free dealuminated HY zeolite produced cracking products with high selectivity for isobutanes ; C₃₊ carbon number distribution was similar to that from CO hydrogenation over dealuminated HY with 2 wt% Ru and 2 wt% Pt. When 1-octene was added into syngas over the various combination of Ru（and/or Pt） catalysts, all the obtained products resulted in similar products pattern besides the i/n ratios of cracking products. Therefore selective formation of isoalkanes from syngas could be essentially due to acid catalyzed reaction on zeolite. With regard to the active center for isoalkane formation modified bifunctional mechanism was also proposed.