© 2017 Springer Science+Business Media, LLC, part of Springer Nature conductivity of Ba 0.72 K 0.28 Fe 2 As 2 single crystal are studied systematically by resistance measurements as a function of temperature and magnetic field. A clear Maki−Thompson and Aslamakov−Larkin (MT–AL) two- to three-dimensional (2D–3D) crossover is found on the excess conductivity (Δσ) curves as the temperature approaches the superconducting critical temperature, T c . 3D fluctuations in superconductivity are realized near T c that are well fitted to experimental data by the 3D Aslamazov–Larkin theory. The Maki–Thompson model shows a 2D conductivity fluctuation above the 2D-3D temperature transition, T 0 , which depends on magnetic field. Results show that the 2D-3D dimensional crossover moves to lower temperature with increasing magnetic field. The values of the transition temperature and the crossover in the reduced temperature, ln(ε 0 ), as functions of magnetic field were used to determine the coherence length and the lifetime, τ φ , of the fluctuational pairs at the temperature of 35 K. Analysis of the Ba 0.72 K 0.28 Fe 2 As 2 single crystal gives a value of 3.76 × 10 − 12 s for the τ φ in the absence of magnetic field and it decreases to 2.4 × 10 − 12 s in magnetic field of 13 T.