Let \(\mu \) be a Radon measure on \(\mathbb {R}^{d}\) which satisfies the growth condition \(\mu (Q(x,l))\le C_{0}l^{n}\) for any cube \( Q(x,l)\subset \mathbb {R}^{d}\) , \(x\in \mathbb {R}^{d}\) and \(l(Q)>0\) , with some fixed constants \(C_{0}>0\) and \(n\in (0,d]\) . We introduce a new type of fractional maximal operator \(M^{a}\) ( \(0\le a<1\) ) under the measure \(\mu \) . If \(\mu (\mathbb {R}^{d})<\infty \) , we obtain a lower oscillation estimate of \(M^{a}f\) and the boundedness of \(M^{a}\) from \(RBMO(\mu )\) to \(RBLO(\mu )\) , which is a generalization of R. Gibara and J. Kline’s result (JFA, 2023) on the measure \(\mu \) . Finally, we obtain some estimates for natural fractional maximal operators on \(RBLO(\mu )\) spaces, which is a generalization of Dachun Yang et al. (LNM, 2017).