A new liquefaction process for natural gas is developed to improve process thermal efficiency while using industrial standard equipment. The new process consists of a pre-cooling section which uses mixed refrigerant with brazed aluminum plate-fin exchangers or a spool wound heat exchanger, and a Liquefaction section which uses an iso-entropic expander. Thermal efficiency of the new liquefaction process is confirmed to be of the highest level compared with other liquefaction processes. Also, since the new liquefaction process is constructed with commonly available industrial equipment, it can be readily adapted to liquefaction plants of any capacity without requiring expensive and specially designed equipment.