Degradation characteristics of kraft lignin were investigated during peracetic acid (PAA) treatment at room temperature. PAA was prepared by direct mixing of acetic acid (AA) and hydrogen peroxide (HP) at different ratios. At a ratio of AA to HP of 1:1.5 (v/v), undissolved lignin content was the lowest (34.7%). Lignin-derived compounds that were produced from the initial lignin after PAA treatment were detected in the liquid fraction (EA extracts) in small quantities (< below 0.1% of initial lignin), while their species were different depending on mix ratio. It was found that degradation behavior depends on not only PAA, but also HP concentrations. Meanwhile, the lignin-derived products of EA extracts in liquid fractions showed decreased molecular weight and polydispersity compared with the initial lignin. As reaction severity increased, amounts of low-molecular weight lignin in liquid fractions increased. At room temperature, different lignin degradation behavior can be induced by controlling the mix ratio of AA and HP.