To understand the influence of different proteins and their additive amounts on permittivities of milk, and to explore a method for detecting the adulteration of milk protein, the open-ended coaxial-line probe technology was used to study the influence of frequency (20~4500MHz) and protein additive amounts (0~4.70%) of different proteins, including whey protein, soybean protein and tripolycyanamide, on permittivities (dielectric constant ε′ and dielectric loss factor ε″) of milk, and the influence mechanism of different proteins and their additive amounts on permittivities were analyzed. The results showed that over the frequency range of 20~4500MHz, ε′ was decreased with the increase of frequency, and ε″ decreased firstly and then increased with the increasing frequency. The minimum value of ε″ observed at around 2000MHz. With the same protein additive amount, the milk added with soybean protein had the largest ε′, while the milk added with tripolycyanamide had the smallest ε′. ε″ of the milk added with whey protein and soybean protein were higher than the values of milk with tripolycyanamide at the same protein additive amount. The permittivity values (ε′ and ε″) of milk were increased linearly with the increase of additive amounts of soybean and whey protein, and all the linear correlation coefficients were greater than 0.92. However, the correlation between the permittivities values and tripolycyanamide content was not obvious, and all the linear correlation coefficients were less than 0.51. The study results indicated that different proteins and their additive amounts had different influence on dielectric properties of milk, which made it possible to identify the different protein types and their additive amounts in milk based on dielectric properties.