The power penalty estimation for fiber optic communication due to mode partition noise in nearly single mode lasers is discussed. Nearly single mode lasers are modeled here as those with a single dominant mode and n side modes with significant strength. The dropout rate of each side mode obeys exponential distribution. The combined distribution of the dropout rate for n side modes is derived here. This distribution is combined with the statistically independent Gaussian receiver noise in estimating error probability and hence the power penalty. Calculations show that to ensure a 10 -12 error rate, the ratio of the main mode to the side modes must be greater than 57:1 for one side mode and greater than 67:1 for two side modes.