The chain growth probability of Fischer-Tropsch synthesis (FTS) was investigated on the basis of data from a set of experiments over an industrial coprecipitated Fe−Cu−K catalyst developed by the Institute of Coal Chemistry (China). The experimental results show that the ASF chain growth probability (α) depends on operation conditions, temperature (T) and H2/CO ratio in the feed. α decreases with T or H2/CO feed ratio increasing. The experimental results also show that chain growth probabilities of hydrocarbons are not constant, but a function of carbon number. This deviation from ASF assumption is further discussed.