Abstract: This paper reports on the preparation and application of the fortified paste size. Data obtained through GC analysis, IR determination and electronmicro-scopic observation all showed the good sizing quality of the fortified paste size. The volume of the rosin particle in the paste size was between 0.1 to 0.2um,the same as similar products in Japan. Using fortified paste size, the expense of rosin and aluminium sulfate can be reduced respectively by 66% and 55% for natural rosin size and 43% and 21% for maleated rosin size in the sizing process. Since fortified paste size is stabilety and somewhat fluidic, it can be stored for a long time (1~2 years) and used conveniently. Practices show that up to now fortified paste size is a very good kind of paper size.