The swelling behavior of lyocell fiber in alkali solutions and the alkali uptake were investigated as well as their influences on the reaction of sodium-hydroxy-dichlor-triazine with lyocell. The uptake of NaOH was increased from 0.69 mmol/g up to 4.63 mmol/g, leading to the enhancement of fiber swelling from 1.01 cm3/g to 2.34 cm3/g when alkali concentration in preliminary alkali treatment was increased from 0.4 mol/l to 8.0 mol/l. The rise in alkali uptake heightened the crosslinking reaction. The fiber swelling was hindered by addition of acetone to alkali solution, resulting in water retention capacity of 0.64 cm3/g in the 37.5% v/v of acetone/water mixture and increase in the reaction yield. The fiber was more swollen in NaOH solution than in KOH though the uptake of NaOH was 5.7-times less than that of KOH. The reaction yield of crosslinking agent in NaOH solution was 9.9-times larger than that in KOH at the same alkali uptake. The abrasion resistance of lyocell fiber was improved by the method used in this work, causing high pilling resistance of lyocell fabric as compared to a conventional method.