Practical applications of the box culvert have been common in many fields, since precast concrete products reduce complicated procedures of situcast concrete at field. The main characteristics of precast product are well controlled processes and guaranteed guality, which are reguired for R.C structures to keep high durability especially in Okinawa prefecture since many damages of R.C structures due to salt are reported recently. The other characteristics is to reduce the weight of box culvert which is performed by introduction of the partially prestressing technigue, i,e., only top and bottom slabs are prestressed by use of unbonded P.C，tendons. Many tests have been conducted by user's reguests, where only crack and failure loads are interested. Therefore, close investigations on structural behaviors up to failure are observed in a few cases. This paper places emphasis on the behaviors by making use of theoretical and test results where deflections, strains, crack loads, ultimate loads and crack patterns are described. The theoretical and test results give the following conclusions: (1) Theoretical values of deflection are quite close to the test results and Branson's equation after crack initiation well coincides with the test values. According to the values of deflection, the reaction with sand cushion could be assumed as uniform distribution. However, strain diaqrams do not yield the same conclusion and it requires more research on the distribution of reaction. (2) Measured strains become much smaller than the theoretical ones, and. consequently, the crack loads become 1.64 times the design load for two box culverts, which indicats sufficient resistance to repeated load. (3) Ultimate strength shows 4 times the design load and it will be concluded that the box culverts are safe enough in practical uses asthe under ground structure., 紀要論文