Chemical reaction occurred in the interface of Mn-Zn ferrite and glass after bonding. Effects of the formation of reaction layer on the magnetic properties were investigated. The composition of glass was 23PbO-61SiO2-6ZnO-8Na2O-2K2O and the ferrite was chosen to have a high permeability. Toroid samples of ferrites bonded with glasses, were heat-treated at $700^{circ}C$, 80$0^{circ}C$ and 90$0^{circ}C$ for 1h. The reaction was observed to increase with bonding temperature, resulting in the development of reaction layer. Subsequently the initial permeability was found to be decreased. The permeabilities decreased by 25% with increasing bonding temperature from $700^{circ}C$ to 80$0^{circ}C$. At the bonding temperature of 90$0^{circ}C$, the permeability was decreased by 45%, compared to that of 80$0^{circ}C$.