Abstract. We have systematically studied and compared the effect of cooling rate on microstructure, critical current density, upper critical field and irreversibility field in pure and 10 wt% SiC-added MgB2 superconductors. The sintering process was carried out on the samples at a temperature of 750 ÃÂ°C for 1 h followed by quenching or cooling to room temperature in 0.3 h (2433 ÃÂ°C h-1), 14 h (52 ÃÂ°C h-1) and 25 h (30 ÃÂ°C h-1). Changes in the microstructure due to variations in cooling rate have been studied with the help of scanning and transmission electron microscopy. Correlations between microstructure and superconducting properties have been observed, identified and explained for both pure and SiC-added MgB2 samples. Modifications to the pinning environment and grain boundary transparency are considered to be responsible for variations in the current-carrying ability. The dominant pinning on grain boundaries in the pure MgB2 samples and on nano-inclusions (inducing accompanying defects) in the SiC-doped samples is clearly distinguished. On the basis of our experimental results, we have concluded that the cooling rate can be an important parameter influencing the superconducting properties of MgB2 samples.