We numerically simulated the dymamics of mode locking in pure positive dipersion lasers by NLS. Different from soliton lasers operating in net negative dispersion regime or dispersion mananged soliton and similariton lasers operationg in net positive dispersion regime with disperion compensation components, gain saturation and narrowing plays an important role in pulse formation in lasers composed of pure positive dispersion medium, in which the pulses evolve to gain-guided solitons. Stable gain-guided soliton was obtain in two kinds of mode locked fiber lasers operating in the positive dipersion regime, and experimental results confirmed the numerical simulation.