Abstract: The discussion of the two meanings of socially necessary labor time in Das Kapital must be combined with the evolution history of commodity exchange. The first meaning is based on the simple commodity economy of barter exchange and the assumption of complete competition. The second meaning is concluded from the history of capitalism socialized mass production in the monetary economy, and on the condition that the supply and demand inequality became an ordinary state. short-term inequality in supply and demand will affect the price and value of commodities while long-term supply and demand inequality tends to balance them under the effect of complete competition. Under the condition that the land structure becomes imbalanced, the total land quantity keeps unchanged, and the land is monopolized in long-term, the value of land commodities per unit is less than the socially necessary labor time in inferior land but greater than the socially necessary labor time in superior land, while the social value of other commodities is less than the social necessary labor time actually paid. Monopoly has a permanent effect on the deviation of unit commodity value from the actual necessary labor time while there is no influence on the change of money demand.

Key words: political economy, socially necessary labor time, supply and demand, competition, evolution