NOVELTY - Preparation of n-butanol involves adding reaction mixture comprising ethanol and acetaldehyde to reactor in reactor feed section, and contacting reaction mixture with Guerbet catalyst in reactor (40). The average mole ratio of acetaldehyde to ethanol in reaction mixture in the reactor feed section is 0.001:1-0.05:1. The acetaldehyde concentration profile in the reactor feed section is such that the mole ratio of acetaldehyde to ethanol in any region of the reactor feed section does not differ by 50% or more from average mole ratio of acetaldehyde to ethanol.    USE - Preparation of n-butanol used in gasoline engine of car.    ADVANTAGE - The method enables preparation of n-butanol having excellent green replacement for gasoline, with high yield of 90-95% and selectivity of 75% or more.    DETAILED DESCRIPTION - Preparation of n-butanol involves adding reaction mixture comprising ethanol and acetaldehyde to reactor in reactor feed section, and contacting the reaction mixture with Guerbet catalyst in reactor at 50-450 degrees C and 10-200 bar. The n-butanol yield based on ethanol is 10-35%. The selectivity to n-butanol is 65-95%. The average mole ratio of acetaldehyde to ethanol in the reaction mixture in the reactor feed section is 0.001:1-0.05:1. The acetaldehyde concentration profile in the reactor feed section is such that the mole ratio of acetaldehyde to ethanol in any region of the reactor feed section does not differ by 50% or more from the average mole ratio of acetaldehyde to ethanol in the reactor feed section. The reactor feed section is defined by reaction mixture addition site (r1) and last reaction mixture addition site located along length of the reactor. The reactor section from reaction mixture addition site (r1) to the last reaction mixture addition site is reactor feed section.    DESCRIPTION OF DRAWING(S) - The drawing shows a flowchart of preparation of n-butanol.    Ethanol feed (1)    Feed stream (15)    Reactor (40)    Molecular sieve (80)    Water rich feed stream (88)