Circulating fluidized bed (CFB) boilers face sever challenges in China due to the new ultra-low emission rules, more specifically, the NOx emission must be lower than 50 mg/m(3). In this paper, NOx evolution inside CFB furnaces and the gas-solid flow characteristics were reviewed. It is found that NOx emission can be reduced through re-specification of fluidization state, namely, improving the bed quality. A low NOx emission (de-NOx) technology applied to CFB boilers was thus proposed. The keys of the technology are decreasing the average size of bed materials, limestone included, and increasing the solid circulating rate by improving the performance of circulating loop including improving cyclone efficiency, controlling slag discharge, etc. Some industrial practices showed that when the average size of bed inventories is markedly reduced, and when using ultrafine limestone, NOx emission can be significantly decreased without undermining high efficiency desulfurization. This paper provides a new solution to low-cost pollution control of CFB boilers.