When two optical fields interact with a four-level N type atom, there are three transitions and the middle transition is used as a probe transition. The two field are called coupled fields and probe field. Two cases of electromagnetically induced transparency (EIT) are discussed. When the frequency changes with time in sinusoidal form, the frequency of electromagnetically induced transparency won′t change but the degree of EIT will change with time periodically. When the frequency changes with time in rectangular form, the frequency will change, the frequency shift can be calculated by the formula proposed in the article. The amplitude of EIT changing with time periodically-energy increases when the modulation amplitude of these rectangular pulses is increased.