A random box-ball system starts with occupying each of the first n boxes independently with a ball of random color from {0, 1, · · · , κ}, where balls of color 0 are considered as empty boxes. The time evolution is defined by a successive application of the combinatorial R, and possesses a κ-tuple of Young diagrams as the complete set of conserved quantities. Using a Markov chain method, we show that if we scale the rows of each of the invariant Young diagrams by 1/n, it converges to some limiting shape as n → ∞ at an exponential rate. Furthermore, we determine the limiting shape by ratios of Schur polynomials with initial ball densities as parameters. We also derive similar results through an alternative method using the Fermionic form and Thermodynamic Bethe Ansatz, which apply once we condition the initial measure on the set of highest states. By a large deviations principle, we identify the limiting shapes of invariant Young diagrams corresponding to the unconditioned and conditioned initial measures.2010 Mathematics Subject Classification. 37K40, 60J10, 60F10.