This study was carried out to study effectiveness of the rare earth element, Lanthanum (La) in the form of Lanthanum Chloride (LaCl3) at two concentrations (1ppm, 5ppm) on controlling The Rose Aphid, Macrosiphum rosae L. (Homoptera: Aphididae) which infesting rose plants (Rosa gallica) through effect on the insect physiology. Experiments were carried out under plastic greenhouses during season 2020/2021 at two locations (Governorates), Al-Zohrya Garden (Cairo Governorate) and Antoniadis Garden (Alexandria Governorate). The results obtained indicated to the clear effect of treated rose plants by Lanthanum Chloride (LaCl3) on the population fluctuation of M. rosae through effect of the insect physiology and also through improving the plant healthy. Whereas the mean number of M. rosae infested rose plants (treated with low concentration of LaCl3) was smaller than the mean number of that insect infested rose plants (treated with high concentration of LaCl3) compared to control (rose plants did not treat with LaCl3). Also study included effectiveness of LaCl3 on the insect physiology (the main target of this study) which seems clear through the effect on the important substances produced by the insect such as total proteins, carbohydrates, total lipids and important enzymes (Chitinase, Lipase, Phosphatase, Kinase, Alpha esterase, Beta esterase, Oxidation enzymes and Digestive enzymes).Whereas the results obtained show those substances decrease in the successive insect M. rosae when fed on rose plants treated with low concentration of LaCl3 (1ppm) compared to control (insects which fed on rose plants did not treat with LaCl3), and vice versa whereas these substances increase in the successive insect M. rosae when fed on rose plants treated with high concentration of LaCl3 (5ppm) compared to control. This study also included effect of treated rose plants with LaCl3 on the internal components of these plants such as total proteins, carbohydrates, total lipids, amino acids, total phenols, tannins and flavonoids, whereas when treated rose plants with low concentration of LaCl3 led to improving the plant healthy through increasing these components compared to control (rose plants did not treat with LaCl3) and vice versa when treated rose plants with high concentration of LaCl3 led to decrease these internal components compared to control.