Charge order has been universally observed in underdoped copper-oxide based high-Tc superconductors (cuprates) and shown to compete with superconductivity. In a strong spin orbit coupled cuprate analogue system, layered perovskite iridium oxides (iridates), Mott insulating, pseudogap and d-wave gap behaviors have been discovered in the single layer variant Sr2IrO4, while metal insulator transition and charge-order like Fermi surface instability has only been seen in the bilayer counterpart Sr3Ir2O7. However, a full symmetry characterization on the charge order in Sr3Ir2O7 is yet missing, making its analogy to the charge order in cuprates incomplete. In this talk, I will show that an amplitude mode of charge order at ~25cm-1 is detected in the metallic La-doped Sr3Ir2O7 at temperatures below ~ 200K by using Raman spectroscopy, which is reminiscent of that observed in cuprates. I will further show that this charge order has two-fold rotational symmetry, breaking the four-fold one of the crystal lattice, which is the same as the unidirectional charge order in cuprates does. Finally, I will discuss the weak coupling between the charge order and the lattice by tracking the phonon evolution upon the emergence of the charge order.