The dielectric properties of Au/Si3N4/n-Si (MIS) structures are studied using the admittance measurements (C-V and G/omega-V) each as a function of temperature in a range from 80 K to 400 K for two frequencies (100 kHz and 1 MHz). Experimental results show that both the dielectric constant (epsilon') and the dielectric loss (epsilon '') increase with temperature increasing and decrease with frequency increasing. The measurements also show that the ac conductivity (sigma(ac)) increases with temperature and frequency increasing. The ln sigma(ac) versus 1000/T plot shows two linear regions with different slopes which correspond to low (120 K-240 K) and high (280 K-400 K) temperature ranges for the two frequencies. It is found that activation energy increases with frequency and temperature increasing.