Analytical study has been carried out to estimate yield strengths of steel beam to box column connections. The obtained solution, which is based on the yield line theory, predicts accurately experimental behaviors and yield strengths for connections both with and without reinforcing diaphragm as follows : 1) For connection without diaphragm, Eq. (9) gives very close approximation to the experimental general yield strength which varies with type of corner weld of column as well as size of column and beam. 2) For connection reinforced with inner diaphragm, Eq. (21) is proposed. Estimated yield strength agrees well with the experimental strength dependent on several sets of parameters, especially, width to length ratio of diaphragm. It is also shown that the test results presented by K. Ban and K. Imai fairly conform to the formula. 3) For connection reinforced with outer diaphragm (or outside stiffening ring), calculated yield strengths by the formula of Eq. (23) are compared with experimental values offered by M. Tabuchi and K. Kanatani et al. As a result, good agreement is observed between the theory and the test results.