The packing of mono-sized spheres under one-dimensional (1D) vibration is studied experimentally. The effects of operational conditions, such as vibration amplitude A and vibration frequency omega, and feeding method on packing density have been analyzed. The results indicate that there exist optimum values for A and omega to achieve the maximum packing density. The effects of A and omega cannot be represented by a single parameter (i.e. vibration intensity Gamma = A omega(2), but should be considered separately. The number of particles fed per batch affects the packing density significantly within a range of one to four layers per batch, but otherwise has no visible effect. Through the extrapolation on packing density using different sized containers, packing density can reach 0.636 in the total feeding method and 0.663 using the batch-wise feeding method. The values, however, are affected by material properties. The experimental results have therefore testified our previous numerical work on the transition from random. loose packing to random close packing [An et al., Phys. Rev. Lett. 95, 205502 (2005)]. (C) 2009 Elsevier B.V. All rights reserved.