Shadow maps provide a fast and convenient method of generating real-time shadows in computer games. Unfortunately, shadow maps suffer from aliasing problems. Percentage Closer Filtering (PCF) is one of the solutions, but in cases of severe aliasing it can only mask the aliasing by unnaturally blurring. In this paper we present two improved antialiased shadow algorithms based on PCF, named as Modified PCF I and Modified PCF II. The primary idea of our algorithms is to make sawtooth shadow edges straighter just by data modification before bilinear interpolation in PCF. We respectively implement the algorithms with graphic hardware in Pixel Shader 2.0 and 3.0, several examples are also provided to show that most of aliasing artifacts have been removed compared with PCF. The time complexity and hardware requirement of Modified PCF I are almost as same as that of PCF. The resource requirement of Modified PCF II is more rigorous than that of Modified PCF I, but it can still accommodate to real-time game rendering with obvious antialiasing effect.