A typical microwave burst with fine structures (type Hand) observed by the Dwingloo multi-channel spectrometer is proposed as evidence of a magnetic tube with a small scale and acceleration of electron beams with a short time scale at the source. It is predicted from the observations and theory that the average velocity of non-thermal electrons is about 0.1c and the density of the beam is about 104 cm−3. There is a micro-magnetic tube with a scale length of six hundred kilometers at the microwave source. There is also a quasi-periodic (~ 20 ms) evolution of the beam energy, which may be accelerated by tearing modes. The spectrum of the burst can be explained by the non-uniformity of the magnetic field. The evolution of the spectrum during a single pulse may result from the electron cyclotron maser instability. Hence, information from the fine structure of the spectrum of solar radio bursts is very important for understanding the mechanism of solar flares.