An adjustable dumbbell device includes a handle bar having two plates extended from end portions and two extensions, a number of weight members each having a groove and a passage, two blocks are secured to the extensions, two pairs of beams are secured between the plates and the blocks and engageable into the groove of the weight member, two latches are slidably engaged onto the extensions, two rods are secured to the latches for moving the latches relative to the beams, two catches are engaged onto the rods and biased to engage with the beams, and two weight elements are detachably attached to the blocks, the weight elements each has a weight of one half (½) of that of the weight member.