In this study, it was observed that the effect of soda ash on fastness properties of reactive dyed cotton fabric was compared with the varying concentration of soda ash. Here, five different concentration of soda ash (10g/l, 15g/l, 20g/l, 25g/land 30g/l) were used to compare the fastness. Different fastness properties of cotton dyed fabric were investigated such as washing fastness, perspiration fastness, rubbing fastness, and light fastness. Most of the cases, the fabric fastness properties such as washing, rubbing, perspiration, and light were improved at soda 20gm/L after further increasing the amount of soda, the fastness properties were same. So, the result of fastness properties at 20gm/L concentration of soda ash was excellent.