It is well known that strain fields alter the electrical and optical properties of semiconductors, via modulating the band structure and lifting band degeneracy. Topological Dirac and Weyl semi-metals belongs to a class of zero bandgap semiconductor, in which the band degeneracy is protected by symmetry or band topology. We have synthesized single crystals of a Dirac semimetal candidate ZrTe 5 , and discovered that its bandgap could be sensitively controlled by strain. In the language of high energy physics we can control the mass of Dirac fermions in this system. We also discovered a method to systematically control the carrier density via crystal growth procedure and chemical substitution, so that the quantum limit, i.e. when all electrons are filled in the lowest Landau levels, can be easily reached within one Tesla. In this talk, I will present our explorations of the strange behaviors of these massive Dirac fermions, using precise control of chemical potential, bandgap and magnetic field.