A natural gas engine based on the principle of a long stroke reciprocating mechanism (LSRM) is proposed. The crankshaft arrangement in a conventional internal combustion (IC) engine will be replaced by a LSRM featuring long stroke and low speed. A major advantage of this LSRM system is that it considerably reduces the side force, which is inherent in conventional crankshaft arrangements. Another advantage is that the LSRM engine will be more compact than an equivalent crank engine operating under the same conditions of speed, power output and so on. The reduced size and complexity of the engine lead to a favorable reduction in manufacturing and, more importantly, maintenance costs. Using gas fuel will allow an increased compression ratio, which is usually limited by the operating nature of the Otto cycle in gasoline engines. With a longer stroke length and lower engine speed, the engine fuel injection system can achieve better engine combustion and performance than current IC engines. This paper reports the preliminary analysis of the LSRM engine. The advantages and anticipated problems are discussed based on a comparison of the LSRM engine and the conventional IC engine. The potential problem of heat loss, which may be caused by the low engine operational speed, was examined through thermodynamic cycle analysis. The results of this analysis show that the heat loss from the LSRM engine will be in the normal range experienced by conventional IC engines if the ambient air is used as the cooling medium.