Abstract: The coolant resistance of glass?fiber?reinforced polyamide 66 (PA 66) was investigated, and the effect of coolant on their mechanical properties was studied. The bending strength and impact strength was evaluated at different aging stage for the PA 66?matrix composite containing 30 wt% glass fiber during 1008?hour thermal aging test in coolant. Moreover, the microstructure was investigated and the corresponding aging mechanism was discussed. The results indicated the materials PA66/30%GF?1 and PA66/30%GF?2 achieved the bending strength higher than 70 MPa along with the retention higher than 25 %. It was concluded that the PA 66 composites with 30 wt% glass fiber supplied by DuPont and Solvay were suitable for long?term use under the coolant condition.