This paper investigates the inverse density segregation of binary particle mixtures under vertical tapping and vibration. Three different sized glass and steel beads (1290, 655 and 463 mum in diameter) were used in the experiment, together with three sizes of vessels (20 cm in height, 3 cm in thickness, and 10, 5 and 3 cm in widths). It has been observed that (i) as a general rule, the smaller the particle, the sharper the segregation of heavier particles to the upper region of the bed, (ii) an appropriate fluidity of the particle bed is essential for the inverse segregation to occur, and (iii) the bed height is an important factor to determine the degree of segregation, given the other conditions.