Velocity transformation is used to change system coordinates in order to simplify the ‎solution of problems that involve relativistic collisions of particles. In this paper I ‎check how the energy of a particle A that moves with some velocity VA‎ ‎ toward an observer ‎ S*‎ is seen by an observer‎ S ‎, under the condition that both observers sees the same ‎energy. I found that although observer in ‎S* ‎ sees particle A with a real rest mass ‎momentum and energy. Observer S‎ ‎ sees two particles. If they move in opposite ‎direction, One of those particles have an imaginary rest mass momentum and energy.‎ I could not decide if the case is only a numerical problem or it is also physical. I ‎expect that those who will read this paper are more experienced then me and will find ‎if the imaginary rest mass is physically true or false. And will suggest how to use it in ‎practical situations.‎