Research is carried out for laser cleaning applied in graffiti removal.With traditional mechanical and chemical methods,there is an obvious risk of over-cleaning and possible damage to the object surface.The laser graffiti removal is a non-contact,easily controlled method that leads to better preservation of the surface.The efficiency of laser graffiti removal versus laser energy density,which is one of the main problems for laser graffiti removal,is investigated through experiments.A high-power high-repetition rate TEA CO2 laser is employed for laser graffiti removal on different substrates and sample surfaces,and experimental data are given.The experimental data have proved that TEA CO2 laser is suitable for graffiti removal;the results are convincible and effective.It is shown that the highest efficiency of graffiti removal and the fastest speed of laser cleaning can be reached for the given samples when laser fluence is between 4 J/cm2 and 6 J/cm2,and the areas of graffiti removed per joule by laser cleaning have been more than 2.5 cm2/J.The main mechanisms involved in laser cleaning,are analyzed and some preliminary conclusions of two main mechanisms are given.When the laser fluence is less than 4 J/cm2,the thermal effects produced by the large absorption coefficient of graffiti materials to the laser energy play an important role.In the range of optimal fluence between 4 J/cm2 and 6 J/cm2,the effects of shock waves produced by plasma detonation rather than the thermal effects are considered as the dominating process.When the laser fluence exceeds 6 J/cm2,the air near sample surface is breakdown,which consumes most of laser energy,thus the removal efficiency decreases significantly,the thermal and mechanical effects are not obvious.