In general all detonation waves have cellular structure formed by the trajectory of the triple points.This paperaims to investigate experimentally the propagation of detonation in narrow gaps for hydrogen-oxygen-argonmixtures in terms of various gap heights and gap widths.The gap of total length 1500 mm was constructed bythree pair of stainless plates,each of them was 500 mm in length,which were inserted in a detonation tube.Thegap heights were varied from 1.2 mm to 3.0 mm while the gap widths were varied from 10 mm to 40 mm.Variousargon dilution rates were tested in the present experiments to change the size of cellular structure.Attempts havebeen made by means of reaction front velocity,shock front velocity,and smoked foil to record variations of cel-lular structure inside the gaps.A combination probe composed of a pressure and an ion probe detected the arrivalof the shock and the reaction front individually at one measurement point.Experimental results show that thenumber of the triple points contained in detonation front decreases with decrease in the gap heights and gapwidths,which lead to larger cellular structures.For mixtures with low detonability,cell size is affected by a cer-tain gap width although conversely cell size is almost independent of gap width.From the present result it wasfound that detonation propagation inside the gaps is strongly governed by the gap height and effects of gap widthis dependent on detonability of mixtures. In general all detonation waves have cellular structure formed by the trajectory of the triple points. This paperaims to investigate experimentally the propagation of detonation in narrow gaps for hydrogen-oxygen-argon mixtures in terms of various gap heights and gap widths.The gap of total length 1500 mm was constructed bythree pair of stainless plates, each of them was 500 mm in length, which were inserted in a detonation tube. The heap were varied from 1.2 mm to 3.0 mm while the gap widths were varied from 10 mm to 40 mm. Variousargon dilution rates were tested in the present experiments to change the size of cellular structure. Attempts havebeen made by means of reaction front velocity, shock front velocity, and smoked foil to record variations of cel-lular structure inside the gaps.A combination probe composed of a pressure and an ion probe detected the arrival of the shock and the reaction front individually individually at one measurement point. Experimental results show that then number of the tri ple points contained in detonation front decreases with decrease in the gap heights and gapwidths, which lead to larger cellular structures. For mixtures with low detonability, cell size is affected by a cer-tain gap width although conversely cell size is almost independent of gap width .From the present result it wasfound that detonation propagation inside the gaps is strongly governed by the gap height and effects of gap widthis dependent on detonability of mixtures.