两者都是形容词，但意思不同。
一、effective表示“有效”，“生效”。如：
His teaching was not very effective. 他的教学效果不好。
The new interest rate becomes effective next month. 新利息下月生效。
When is the contract effective from? 合同什么时候开始生效?
This medicine, taken in time, can be very effective. 这种药如果及时吃会很有效。
My headache is much better. Those tablets really are effective. 我的头疼好多了，那些药片的确很有效。
二、efficient表示“效率高的”，可指人：
He needs an efficient secretary. 他需要一个高效率高。
The staff was very efficient. 工作人员效率很高。
A good manager is both competent and efficient. 一个好的经理既要胜任又要效率的秘书。
也可指机器等：
The diesel engine is highly efficient. 这台柴油机效率很高。
Our efficient new machines are much cheaper to run. 我们高效率的新机器开起来要便宜得多。