De-ionized water，used as working fluid，flows through in-line and staggered array micro/mini-cylinders-group plates with 5.2 mm width and 40 mm length，which are made up of micro-cylinders with hydraulic diameter of 0. 5 mm and heights of 0.5 mm，0.75 mm and 1.0 mm respectively，and the influence of dimension effect on forced convective heat transfer characteristics is studied experimentally. Using an electricity heater，the flow rate and the temperature difference between the inlet and the outlet of micro/mini-cylinders-group are measured and the Nusselt numbers are obtained over low Reynolds numbers ranging from 8 to 100. The experimental results indicate that the Nusselt number for H = 0.5 mm is less than that of classical correlation and it is also less than the Nusselt number for H = 0.75 mm and H =1.0 mm due to the appearance of the sluggish regions in micro/mini cylinders and combined effect of conjugate heat transfer，endwall effect，variation of thermophysical properties with temperature， etc . With the increase of Reynolds number，the Nusselt number of micro/mini-cylinders-group with height of 0.5 mm rapidly increases and it is larger than that with height of 0. 75 mm and 1.0 mm. For Re >20，