Objective To study the impact of imported filariasis cases on the elimination program in different areas of Shandong Province. Methods Dezhou was selected as former low endemic area and Yicheng as former high endemic area. Blood examination was carried out for both mobile population and local people for microfilariae(Mf). Mosquitoes were caught in field and dissected to count the ratio of those having laid eggs and the natural filarial infection rate. Mosquitoes reared at different temperatures were fed with Mf-positive blood and dissected after certain time period to observe the development of the larvae. The vectorial capacity and case transmission quantity were calculated and compared with those from different areas. Results The Mf positive rate of inflow population was 3.18% in average. No case was detected from 9 411 local residents after blood exam in Dezhou while 2 out of 692 local residents were found Mf positive in Yicheng. Mosquitoes'natural infection rate was 3.81% but no third stage larva was found. The shortest time period needed for the larva to develop into an infective stage was 16 days in Dezhou and 11 days in Yicheng. The time period from blood meal to egg-laying on average was 4. 95 days in Dezhou and 4.33 days in Yicheng. The ratio of vectorial capacity and case transmission quantity was 1:4.41 and 1:5. 82 respectively in Dezhou and Yicheng. Conclusion Filarial transmission seems unlikely in Dezhou for its low vectorial capacity and low transmission quantity resulted from low and evidently fluctuating temperature in the north. A low level filarial transmission may be possible in former high-endemic area such as Yicheng if there are as many imported cases as in Dezhou.