Abatement of greenhouse gas emissions and diversification of energy sources will probably lead to an economy based on hydrogen. In order to evaluate safety conditions during transport and distribution, experimental data is needed on the detonation of Hydrogen/Natural gas blend mixtures. The aim of this study is to constitute detonation and deflagration to detonation transition (DDT) database of H2/CH4-air mixtures. More precisely, the detonability of such mixtures is evaluated by the detonation cell size and the DDT run up distance measurements. Large experimental conditions are investigated, (i) various equivalence ratios from 0.6 to 3, (ii) various H2 molar fraction x ( ( )2 2 4x H H CH= + ) from 0.5 to 1, (iii) different initial pressure P0 from 0.2 to 2 bar at fixed ambient temperature T0=293 K. Detonation pressures P, velocities D and cell sizes ? were measured in two smooth tubes with different i.d. d (52 and 106 mm). For DDT data, minimum DDT run up distances LDDT were determined in the d=52 mm tube containing a 2.8 m long Schelkin spiral with a blockage ratio BR = 0.5 and a pitch equal to the diameter. Measured detonation velocities D are very close to the Chapman Jouguet values (DCJ). Concerning the effect of , detonation cell size ? follows a classical U shaped- curve with a minimum close to =1 and concerning the effect of x, ? decreases when x increases. The ratio ik L?= obtained from different chemical kinetics (Li being the ZND induction length) is well approximated by the value 40 in the range 0.5 < x < 0.9 and 50 for x 0.9. Minimum DDT run up distance LDDT varies from 0.36 to 1.1m when x varies from 1 to 0.8. The results show that LDDT obeys the linear law LDDT ~ 30-40?, previously validated in H2/Air mixtures. Adding Hydrogen in Natural Gas promotes the detonability of the mixtures and for x 0.65 these mixtures are considered more sensitive than common heavy Alkane-Air mixtures.