For the tetrainterstitial agglomerate (I4), four additional silicon (Si) atoms are incorporated in an ordinary unit cell of Si lattice in such a manner that all atoms are four-coordinated and angles between bonds are not disturbed significantly. Microstructure, electrical properties, and formation mechanism of I4 in Si were inferred from the investigation of associated electron spin resonance (ESR) spectra in various samples under various experimental conditions. It was found that the B3 ESR spectrum is related to a positive charge state of I4 and the NL51 spectrum is related to the excitonic state of the neutral defect. In Si samples predoped with hydrogen, formation of tetrainterstitial agglomerates after electron-irradiation and annealing is associated with various reactions of hydrogen with intrinsic defects.