Laminar flame speeds and Markstein lengths for n-butanol, s-butanol, i-butanol and t-butanol at pressures from 1 atm to 5 atm were experimentally measured in a heated, dual-chamber vessel. Results at all pressures show that n-butanol has the highest flame speeds, followed by s-butanol and ibutanol, and then t-butanol, which quantitatively agree reasonably well with the computed results using the recent mechanism of Sarathy and co-authors. Results further show that while the isomers have different Markstein lengths, they have similar Markstein numbers which is the appropriate nondimensional parameter to quantify flame stretch. Investigations on thermal effects, reaction rate sensitivities, intermediate species distributions and reaction paths subsequently demonstrate that kinetic effect is the primary reason for the ordering of the flame speed. Specifically, since s-butanol, i-butanol and t-butanol all have branched molecular structures, they crack into relatively stable branched intermediate species, such as iso-butene, iso-propenol and acetone, with the resulting flame speeds depending on the extent of fuel molecule branching.