An adjustable structure for rear seat of bicycle has a rear seat, a clamp, and a connection member. The rear seat and the clamp are respectively arranged a first teeth portion and a second teeth portion. The connection member has a third teeth portion and a fourth teeth portion. The first teeth portion and the third teeth portion are engaged with each other and the second teeth portion and the fourth teeth portion are engaged with each other. The first, second, third, and fourth teeth portions are allowed a little rotation. It may adjust position without totally releasing the screws. It may be quickly positioned.