Objective To study the effect of heat shock protein 40(HSP40) and heat shock protein 70(HSP70) on the aggregate formation of mutant huntingtin (htt) and its toxicity in N2a cells. Methods The effect of the over-expression of HSP40 and HSP70 alone or co-over-expression of them on aggregate formation of transfected mutant htt (containing 150 glutamine repeats, 150Q) in N2a cells was detected by fluorescence microcopy and Western blotting. Cell viabilities were detected by MTT assay. Reactive oxygen species (ROS) production was detected by colorimetric method. Results The over-expression of HSP40 and HSP70 alone, especially the co-over-expression of them, dramatically decreases the formation of 150Q htt aggregates in N2a cells. The numbers of aggregates in each group are as follows (EM>n/EM>=1 000): about 50% in the group of 150Q htt-expressing alone, about 12% in the group of overexpression of HSP40, about 15% in the group of overexpression of HSP70, about 5% in the group of co-over-expression of HSP40 and HSP70. The result of MTT assay shows that the over-expression of HSP40 and HSP70 alone, especially the co-over-expression of them, dramatically increases cell viabilities (EM>P/EM><0.01, EM>n/EM>=3) and reduces the production of ROS (EM>P/EM><0.01, EM>n=/EM>3). Conclusion HSP40 and HSP70 can increase cell viabilities of 150Q N2a cells via inhibiting the aggregates formation of mutant huntingtin and reducing oxidat