The discovery of perpendicular magnetic anisotropy(PMA) in Ta/CoFeB/MgO film and the demonstration of high performance perpendicular magnetic tunnel junction(p-MTJ) based on this material system have accelerated the development of the next-generation high-density non-volatile memories and other spintronic devices. Currently it is urgently needed to improve the interfacial PMA and thermal stability of the CoFeB/MgO system for practical applications. So far, the perpendicularly magnetized CoFeB/MgO films and the corresponding p-MTJs have been extensively explored with the B content of the CoFeB layer mostly fixed at about 20 atomic percent. In this paper, four sets of multilayered films Ta/(Co0.5Fe0.5)1-xBx/MgO(x=0.1, 0.2, 0.3) and MgO/(Co0.5Fe0.5)0.7B0.3/Ta with different CoFeB thickness are deposited on thermally oxidized Si substrates by magnetron sputtering at room temperature, and subsequently they are annealed in high vacuum at different temperatures ranging from 573 to 623 K. The room temperature magnetic properties of the annealed samples are characterized by using vibrating sample magnetometer and superconducting quantum interference device magnetometer. With normal B content of 20% for the CoFeB layer, the Ta/CoFeB/MgO structure annealed at 573 K shows perpendicular magnetization when the CoFeB layer is no thicker than 1.2 nm. As the B content decreases to 10%, it has been found that PMA is achieved only in the sample with a 0.8 nm CoFeB layer under the same annealing condition. The result shows that the interfacial PMA appreciably falls off when the B content is reduced by half. On the other hand, when the B content of the CoFeB layers increases from 20% to 30%, the Ta/CoFeB/MgO structure annealed at 573 K exhibits PMA with the CoFeB layer as thick as 1.4 nm and the interfacial PMA(Ks) increases from 1.710-3 Jm-2 to 1.910-3 Jm-2 together with slightly improved thermal stability. Most remarkably, the MgO/CoFeB/Ta structure with 30% B shows optimum annealing temperature of about 623 K, at which Ks reaches 2.010-3Jm-2 and PMA is realized in the samples with the CoFeB thickness up to 1.5 nm. In contrast, the same structure with 20% B is magnetically destroyed completely under this annealing temperature. The present results suggest that the CoFeB layer with excess B can effectively improve the perpendicular magnetic properties and thermal stability for the Ta/CoFeB/MgO system, and one should take into account the B content effect to optimize the spintronic devices based on the perpendicularly magnetized CoFeB/MgO system.