Metalaxyl fungicide under trade name vacomil 35% wettable powder (WP) was applied on tomato at the recommended dosage 262.5 g a.i./ ha to study the residues and dissipation rate of tested fungicide under field condition by LCMS/MS. Also, the tested fungicide was stored according to Food and Agriculture Organization (FAO) for 14 days at 54 ± 2ºC to study the effect of thermal storage on degradation of metalaxyl active ingredient and stability of their relevant impurities (2,6-dimethylaniline) beside its physical properties. Data illustrated that residues of metalaxyl reached below the maximum residue limit (MRL) of metalaxyl after 7 days from application at the recommended dosage. The half-life (t 1/2 ) for metalaxyl degradation was 1.84 days. Metalaxyl active ingredient was slightly affected with thermal storage and become conformity with FAO specifications and half-life (t 1/2 ) for metalaxyl active ingredient was 8.99 days. 2,6-dimethylaniline as relevant impurities of metalaxyl was increased by extending storage time. It became non conformity with FAO specification under thermal storage where, it was 0.024% before one day of storage and after 14 days of storage reached 0.042% recording increasing rate 75%. All physical properties tests passed successfully and became conformity with FAO specifications.