Magnetars have a magnetic field up to 10^15 G. The ultra-strong magnetic field has profound implications for the mechanical, thermal and radiative evolution of magnetars. In this talk, I will discuss the conversion of magnetic energy to radiations inside and outside magnetars. Inside the magnetar, we studied the interaction between the magnetodynamics and the mechanical responses of the crust. Internal Hall waves launched from the core-crust interface can develop large magnetic stress that initiate plastic deformation of the crust and power the observed X-ray outbursts. Outside the magnetar, we studied how Alfven waves is dissipated in the magnetosphere. Using a high order simulation of Force-Free Electrodynamics (FFE), we found that the turbulent dissipation of Alfven waves is slow compared to the absorption of waves by the magnetar crust. However, FFE is not applicable when magnetic dominance (magnetic field larger than the electrical field) is lost. In that situation, we demonstrated with kinetic simulations that a transient current sheet is formed with an increased gradient of the local magnetic field. Magnetic reconnection can be triggered in 2D which accelerates particles and provides a new channel of dissipation.