Miniaturization of the electronic devices has steadily reduced both the size and the cost. But a power density of battery has not increased a lot. Batteries are a prime agent to penetrate and expand of ubiquitous life. But their intrinsically low energy densities and short lifetimes impose a fundamental limitation. Due to this reasons, a small linear reciprocating engine and electric generator are designed for a portable power pack in this paper. If an engine is used solely to produce electrical power in a portable power pack, it is not necessary to convert the reciprocating linear motion of the piston to rotary motion. Instead, a linear electrical generator may be coupled directly to the piston for a conversion of mechanical energy into electrical energy. Such a design has many advantages such as mechanical simplicity and low friction and it can make possible a small engine having high efficiency.