The paper offers a phenomenological model of volume self-diffusion and interstitial diffusion at high (T > T-D) and low (T < T-D) temperatures (where T-D stands for the Debye temperature). Diffusion mechanisms at high and low temperatures were shown to differ greatly. Diffusion at high temperatures occurs as a result of fluctuations that can be described in the localized melting terms - 'liquid diffusion corridor' formation. At low temperatures when melting is difficult for a number of reasons, diffusion occurs through a 'hollow diffusion corridor' formed by fluctuation. Activation energy calculations for self-diffusion agree well with the T > T-D experiment and show a dramatic increase in the activation energy at T < T-D. Interstitial diffusion activation energy calculated for BCC metals agrees well with the experiment of the whole temperature range and helps to explain why diffusion activation energy goes down at low temperatures.