We study the Cauchy problem of the incompressible micropolar fluid system in R 3 . In a recent work of the first author and Jihong Zhao [30], it is proved that the Cauchy problem of the incompressible micropolar fluid system is locally well-posed in the Fourier-Besov spacesḞ B 2− 3 p p,r for 1 < p ≤ ∞ and 1 ≤ r < ∞, and globally well-posed in these spaces with small initial data. In this work we consider the critical case p = 1. We show that this problem is locally well-posed inḞ B −1 1,r for 1 ≤ r ≤ 2, and is globally well-posed in these spaces with small initial data. Furthermore, we prove that such problem is ill-posed inḞ B −1 1,r for 2 < r ≤ ∞, which implies that the function spaceḞ B −1 1,2 is sharp for well-posedness. In addition, using a similar argument we also prove that this problem is ill-posed in the Besov spaceḂ −1 ∞,r with 2 < r ≤ ∞.2010 Mathematics Subject Classification. 35A01, 35Q35, 76D03.