A method for controlling a refrigerant composition in a pre-cooled mixed refrigerant cycle of liquefied natural gas production, wherein controlling the refrigerant composition is carried out in view of hourly weather forecast temperatures provided by a daily weather forecast providing weather forecast temperatures for every hour in advance for 24 hours, wherein the method comprises the steps of: measuring a concentration of each PMR component; measuring an ambient temperature; calculating a required concentration of each component for each of the measured and weather forecast temperatures on the basis of a predetermined relationship between an optimum concentration of each component and the ambient temperature; calculating a time period required to appropriately change each component concentration in transition to a following of the weather forecast temperatures by using a highest possible change (increase or decrease) rate of each component concentration for the said cycle; the method further comprising the steps of: selecting time periods from the calculated periods, which exceed the period between two adjacent weather forecast temperature, that is, the periods exceeding 1 hour; comparing the measured concentration and the required concentration of each component, where the comparison with the required concentration is carried out by using either the a lower or upper permissible limit of the required concentration; and varying each component concentration in view of the selected time periods and a direction of changing the component concentrations during these periods, wherein an excess of the selected time periods over the time period between two adjacent weather forecast temperatures is calculated, and wherein measuring the relevant component concentration is started in advance with respect to the said excess as well as to the direction of the component concentration change. measuring a concentration of each PMR component; measuring an ambient temperature; calculating a required concentration of each component for each of the measured and weather forecast temperatures on the basis of a predetermined relationship between an optimum concentration of each component and the ambient temperature; calculating a time period required to appropriately change each component concentration in transition to a following of the weather forecast temperatures by using a highest possible change (increase or decrease) rate of each component concentration for the said cycle; the method further comprising the steps of: selecting time periods from the calculated periods, which exceed the period between two adjacent weather forecast temperature, that is, the periods exceeding 1 hour; comparing the measured concentration and the required concentration of each component, where the comparison with the required concentration is carried out by using either the a lower or upper permissible limit of the required concentration; and varying each component concentration in view of the selected time periods and a direction of changing the component concentrations during these periods, wherein an excess of the selected time periods over the time period between two adjacent weather forecast temperatures is calculated, and wherein measuring the relevant component concentration is started in advance with respect to the said excess as well as to the direction of the component concentration change. selecting time periods from the calculated periods, which exceed the period between two adjacent weather forecast temperature, that is, the periods exceeding 1 hour; comparing the measured concentration and the required concentration of each component, where the comparison with the required concentration is carried out by using either the a lower or upper permissible limit of the required concentration; and varying each component concentration in view of the selected time periods and a direction of changing the component concentrations during these periods, wherein an excess of the selected time periods over the time period between two adjacent weather forecast temperatures is calculated, and wherein measuring the relevant component concentration is started in advance with respect to the said excess as well as to the direction of the component concentration change.