Chrysanthemum (Chrysanthemum Morifolium Tzvel) is an important medical herb that is widely planted in China. It can not only cure some diseases as a traditional medicine but may be a useful health protective ingredient. It is usually processed into dry flower or health beverage to go on the market. Aphids (Aphids gossypii Glover and Pyrethrmyzus sanborni Gilletti) and stem borers (Phytoecia rufiventris Gautier) are two important pests that damage Chrysanthemum plants and affect the yield of Chrysanthemum flowers. Dimethoate, O,O-dimethyl-S-(Nmethylcarbamoyl-methyl)phosphorodithioate, is a broad-spectrum systemic insecticide used on many crops world-wide. It is also widely used to protect chrysanthemums from Aphids (Aphis gossypii Glover and Pyrethrmyzus sanborni Gilletti) and stem borer (Phytoecia rufiventris Gautier) in China currently. Because high residues of dimethoate will affect human health, degradation and residues of dimethoate in crops, fruits and vegetables have been brought to people’s attention. Lee and Westcott (1981) described the rapid degradation of dimethoate in wheat leaves. MacNeil et al.( 1975) studied the persistence of dimethoate and dimethoxan on cherries and found the half life in sweet and sour cherries was 6.6 and 4.69 days. Zwick et al.( 1977) also determined the residues of dimethoate and dimethoxon on sweet cherries and found the half life degraded residues was 3.5 to 4 days. Woodham et al. (1974a, 1974b) designed several experiments for searching the effect of different application on persistence of dimethoate and its oxygen in citrus and showed higher residues by the aid of ultra low volume spray than conventional high volume spray. Yutaka Iwata et al. (1981) reported half life of dimethoate in citrus leaves was 3.6 days. Steller and Brand (1974) reported half life of dimethoate in grapes was 6.8-7.0 days. Ferreira et al. (1987) studied residues of dimethoate and omethoate in peaches and apples and described only in the case of apples with seven applications, mean residues of dimethoate exceeded 1 ppm at the end of the preharvest interval (14 days). Szeto et al. (1985) reported that after application of dimethoate, the residues in asparagus decreased by about 90% in 7 days. There is, however, no report on dimethoate residues in chrysanthemum flowers to be published. To determine dimethoate residues in chrysanthemum products and keep