  Let $\Gamma$ denote a distance-regular graph. The maximum size of codewords
with minimum distance at least $d$ is denoted by $A(\Gamma,d)$. Let $\square_n$
denote the folded $n$-cube $H(n,2)$. We give an upper bound on $A(\square_n,d)$
based on block-diagonalizing the Terwilliger algebra of $\square_n$ and on
semidefinite programming.The technique of this paper is an extension of the
approach taken by A. Schrijver \cite{s} on the study of $A(H(n,2),d)$.
