A simple assembly line balancing problem is considered provided that number m of working stations is xed. In such a problem, denoted as SALBP-2, it is necessary to minimize a cycle time for processing a partially ordered set of assembly operations V = f 1 ; 2 ;:::;n g on a set of m linearly ordered working stations. An initial vector of the processing times t = ( t 1 ;t 2 ;:::;t n ) of the (assembly) operations V is known before solving the problem SALBP-2. For each an automated operation i 2 V n e V , the processing time t i cannot vary during a life cycle of the assembly line. For a subset e V V of the manual operations j 2 e V , the processing times t j may vary, since di erent operators (workers) may have di erent skill, experience, etc. We investigate a stability of the optimal line balance of the simple assembly line with respect to simultaneous variations of the processing times t j of the manual operations j 2 e V . An optimal line balance is stable if its optimality is preserved for any su ciently small variations of the processing times t j , j 2 e V . We propose an enumerative algorithm and a program in C++ for constructing all the stable optimal line balances for the problem SALBP-2. Computational results for benchmark instances have been presented.