The effects of various kinds of micro-defects on minority carrier lifetime in high purity dislocation-free silicon single crystals were studied by float-zoning, photoconductivity attenuation and copper decoration combined with selective etching, X-ray topography and electron-beam-induced-current analysis. We found that swirl defects (A or B type) and unidentified frozen-in defects are responsible for limiting minority carrier lifetimes in high purity silicon crystals. The A-type defect was found to be a recombination center with an effective diameter of approximately 30 – 40 μm. By altering crystal growth conditions such as growth rate and diameter, various thermal gradients and cooling rates were attained. These affected the micro-defect characteristics and hence the lifetimes of the crystals grown.