We propose a kind of photonic crystal fiber with flat-top fundamental mode by introducing a depressed central dip into the core of the conventional index-guiding photonic crystal fiber. The design guidelines and characteristics of the large flattened mode photonic crystal fiber (LFM-PCF) are discussed in detail. By appropriate design, the effective area of the LFM-PCF can be increased by a factor of greater than 2 as compared with conventional index-guiding photonic crystal fiber with the same hole and pitch parameters. The improved effective area, single mode operation and flat-top fundamental mode output make LFM-PCF an ideal candidate to realize high power, high beam quality fiber amplifiers and lasers.