The effect of tempering temperature on microstructure and mechanical properties of 23CoNi steel plate has been studied. Tempering in the temperature range of 100-460℃, the ultimate tensile strength σb tends to change along a curve which has the shape of the letter "n", the peak of the σb occurs in the range of 460-475℃, beyond the peak, σb deceasing as the temperature increasing. Tempering in the temperature range of 100-200℃, the impact property αku increases as the temperature goes up. After 200℃, αku goes down as the temperature increases, when the tempering temperature goes higher than 460℃, αku goes up again tempering at higher than 485℃, the acicular alloying carbides M2C precipitate from matrix. The orientation relationship between M2 C and matrix coincides with follows: (011)α//(001)M2c, [100]α//[100] M2c.We found out a new order phase B2 which dispersing in the matrix and the size is very small. This paper discusses the reason of the formation of this new order phase and the sequence of effects to mechanical properties. With the increasing of the tempering temperature, both MaC and new order phase growing up. As the tempering temperature goes beyond 470℃, the amount of retained austenite tends to go up, higher than 510℃, the growing up of reverted austenite is accelerative. It is identified that retained austenite is coming from the reverse transformation. Tempered at higher than 550℃, we can easily find the reversion austenite which contains much more Nickel than matrix does and emerged in the bulk form under TEM.