Recently, one of the greatest concerns of the world wide pulp and paper industries has been organo-chloro compounds discharged from bleach plants which use chlorine based bleaching chemicals. Although there are some debates on toxicities of organo-chloro compounds, we can find a distinct tendency in developing kraft pulp bleaching technologies which are facing toward less-chlorine or non-chlorine bleaching.Among them, ozone has been recognized as a powerful delignifying reagent. However, one of the drawbacks of ozone bleaching is considered to be its non-selective degradation of cellulose, which leads to the viscosity drops of the pulps. In this situation, it was revealed by Union Camp corporation using high consistency ozone bleaching that even if the pulp viscosities were reduced during the bleaching, the pulps maintained acceptable strength properties, but the values of the viscosities were not shown by the authors.To confirm the relationship of viscosity drops and strength properties, ozone bleaching of kraft pulps was carried out using a medium consistency mixer. Hardwood kraft pulps (LKP) and oxygen-bleached hardwood kraft pulps (LKOP) were successfully bleached to the kappa number below 3 keeping the viscosity above 20 mPa.swith 1% ozone charge.Softwood kraft pulp (NKP) and oxygen-bleached softwood kraft pulp (NKOP) could also be bleached to the kappa number below 4 keeping the viscosity above 13 mPa.s with excess ozone charge. It was also shown that ozone-bleached pulps kept enough strength properties compared to those of the chlorine-bleached pulps. Furthermore, even the pulps which were bleached with an excess amount of ozone maintained the acceptable strength properties although the viscosities were unacceptable level.On the other hand, the severe viscosity reduction could be protected with addition of methanol and the viscosities were kept above 30mPa-s. However, no advantage in strength properties over the non-protected pulps was found.From these results, we consider that the relationship between pulp viscosities and the strength properties of ozone-bleached pulps differs from that of chlorine-bleached pulps. Therefore, it seems that ozone is capable of bleaching kraft pulps without addition of any cellulose protector.