In this paper, the drinking water biotic safety of particles and bacteria attached to fines in activated carbon process was investigated by actual treatment process and advanced treatment pilot trial with granular activated carbon. In the experiment, the particles were detected by IBR particle calculating instrument, the activated carbon fines were counted on the basis of the most probable number (MPN) with a microscope, the total number of bacteria was analyzed between the conventional agar culture medium and the one with R2A, and the bacteria attached to activated carbon fines was resolved by the homogenization technique. The experimental results showed that the average total number of particles was 205 CNT/mL in the activated carbon effluent during a filter cycle, of which the number of particles with sizes >2 μm was 77 CNT/mL more than the present particle control criterion of the American drinking water product standard (50 CNT/mL). The backwash of low density and long duration lowered particle number in the effluent. The MPN of activated carbon fines in the effluent was between 400 and 600 CNT/L, which accounted for less than 5 0 of the total particles from activated carbon filtration for a poor relative level ( = 0.34). The microorganisms in activated carbon effluent consisted mostly of heterotrophic bacillus and the total bacteria number was five times as high as that of the inflow, i.e. the effluent from sand filter. The actual bacteria number may be truly indicated by the detection technique with R2A culture medium compared with the traditional agar cultivation. The inactivation efficiency of bacteria attached to activated carbon fines was less than 40% under 1.1 mg/L of chlorine contacting for 40 min. Results showed that the particles and bacteria attached to activated carbon fines may influence drinking water biotic safety, and that the effective control measures need to be further investigated.