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To offer a method for detecting protein content in milk, raw fresh milk and soy protein powder were used to study the influence of frequency (20~4 500 MHz), temperature (5~75℃) and soy protein mass fraction (0~3.98%) on dielectric properties (dielectric constant ε′ and dielectric loss factor ε″) with vector network analyzer and open-ended coaxial-line probe. The models describing the relationship between ε′ and ε″ with temperature and soy protein mass fraction at 40 MHz were established. The results showed that ε′ decreased with increase of frequency and temperature. ε″ had minimum values at 1 000~4 000 MHz, and it increased with the increase of temperature when frequency was below 1 000 MHz and decreased with temperature increase when frequency was above 4 000 MHz. Both ε′ and ε″ increased linearly with increase of soy protein mass fraction. Two-parameter second-order polynomials could be used to describe the relationship between ε′ and ε″ with temperature and soy protein mass fraction. The determination coefficients of developed polynomials at 27 MHz were 0.992 and 0.998, respectively. The result is helpful to understand the influence of frequency, temperature and soy protein mass fraction on dielectric properties of raw milk, and it offers a basis for developing protein content detector for milk.