The difference in electrochemical hydrogen storage of pure multi-walled carbon nanotubes (MWNTs) with different diameters of 10-20 nm, 10-30 nm, 20-40 nm, 40-60 nm and 60-100 nm were investigated. The MWNTs were synthesized by chemical vapor deposition (CVD) using LaNi5 alloy particles as catalyst and were treated in vacuum at 600. Three-electrode system was introduced. The CNTs-Ni electrodes were used as the working electrode, which were prepared by mixing carbon nanotubes (CNTs) and Ni powder in a weight ratio of 1:9 and compressing the mixture into porous nickel collector. Ni(OH)2/NiOOH worked as the counter electrode and Hg/HgO as the reference electrode. 6 mol/L KOH solution acted as the electrolyte. Results showed that MWNTs with different diameters represented a great discrepancy in the electrochemical hydrogen storage capability under the same testing condition. 10-30nm CNTs has the best electrochemical hydrogen storage capacity with a highest capacity of 480.6 mAh/g and a corresponding discharging plateau voltage of 0.95 V, which were gained at 200 cycle with 200 mA/g charge/discharge current density. However, 60-100nm CNTs has the lowest capacity of 298.3 mAh/g under the same testing condition. This shows that the tube diameter of CNTs is an important factor that influences its electrochemical hydrogen storage performance.