Seventy six isolates of Phytophthora infestans from North-West China was determined for the mating type, and an effective preservation method using mineral oil was developed. All P. infestans isolated had A1-mating type. When the isolated were stored in mineral oil, 94.9% of P. infestans isolates were viable. The composite fitness index of P. infestans isolates preserved in mineral oil accounted for 14,806~119,935 sporangia per cm2, with 40,926 sporangia per cm2 for the average composite fitness index. All data suggest that mineral oil preservation for P. infestans can be an easy and efficient preservation method for long term storage of P. infestans without losing pathogenicity.