Data on the critical parameters of initiation of okfol-3.5 mixtures with aluminum powder of different dispersion (0.1–160 μm) obtained using weight-drop machine experiments is presented in the article. The purpose of this work is to determine the following: is there a dependence of the sensitivity of the binary mixture of the investigated explosive on the particle size of the metal additive with components different concentration. Sensitivity data for explosive mixtures are important in ensuring safety of storage, transportation, and production. An attempt was also made to answer the question: by what mechanism does initiation occur (in connection with the chemical reaction or due to the particles frictional heating). When comparing the results, it is obvious that the sensitivity of mixtures with micro-sized aluminum is almost the same within the accuracy of the experiments. At the same time, mixtures in which nanosized aluminum was used appear to be somewhat more sensitive to impact. The greatest impact sensitivity is manifested when the concentration of micron aluminum powder is in the range of 20–30 %, while when using nano-dispersed aluminum, a pronounced minimum is not observed. The comparison of data obtained with previously published for determination of the main mechanism of mixtures sensitization: frictional heating of active particles or chemical interaction of the decay products of explosives with aluminum. The values of the critical initiation pressures presented in the work are in qualitative agreement with previously published data, however, the quantitative discrepancies are identified. When comparing the initiation pressures of octogen-Al and octogen-Al₂O₃ mixtures the convergence of the results was revealed, that allows to draw a conclusion about the decisive contribution of the friction mechanism of explosion initiation during mechanical initiation of the metallized mixtures.