Abstract: Water soluble carbon quantum dots (CQDs) were prepared from peel. Transmission electron microscopy (TEM) analysis showed that the average particle size of CQDs was 2.4 nm. FTIR spectroscopy revealed that the surface of CQDs was rich in oxygen-containing functional groups. The obtained CQDs can catalyze the H2O2-mediated oxidation of 3,3',5,5'- tetramethylbenzidine (TMB) to produce corresponding blue oxidation product (ox-TMB). Thus, a simple method for the detection of H2O2 was established. Under the conditions of pH 3.5, temperature 40°C, TMB 1.0mmol/L and incubation time 15 min, the absorbance of ox-TMB increased linearly with increasing H2O2 concentration in the range of 5.0~100 μmol/L, the detection limit for H2O2 was 3.0 μmol/L. Given that H2O2 was one of the products of sarcosine oxidase (SOX)-catalyzed sarcosine oxidation, a method for the indirect detection of sarcosine based on CQDs-catalyzed and H2O2-mediated TMB oxidation was also developed. The increase of ox-TMB absorbance enabled the sensitive determination of sarcosine in the range of 0.5~350 μmol/L, and the detection limit (3σ/k) for sarcosine was calculated to be 0.3 μmol/L. The method exhibited its potential application in the detection of sarcosine in human urine samples with the recovery of 96 %～99 %.