The present invention relates to a method for producing liquid or solid hydrocarbons from a synthesis gas via Fischer-Tropsch synthesis which does not carry out a separate reduction pre-treatment for catalyst activation. The method for producing liquid or solid hydrocarbons from a synthesis gas using Fischer-Tropsch synthesis according to the present invention comprises: a first step of applying an iron-based catalyst for the Fischer-Tropsch synthesis in which the number of iron atoms in the ferrihydrite phase fraction equals 10 to 100% and the number of iron atoms in the hematite phase fraction equals 0 to 90%, with respect to 100% of the number of the number of iron atoms, to a Fischer-Tropsch synthesis reactor; and a second step of activating the catalyst for the Fischer-Tropsch synthesis by a synthesis gas which is a reactant under the conditions of the Fischer-Tropsch synthesis and carrying out the Fischer-Tropsch synthesis by means of the activated catalyst for the Fischer-Tropsch synthesis. As such, the present invention is capable of efficiently producing liquid or solid hydrocarbons from a synthesis gas via Fischer-Tropsch synthesis, even without a separate reduction pre-treatment.