Why is a dc line an efficient transmitter of power as compared to an ac line ?

No skin effect for DC, so less loss

DC is more efficient than AC because there is no losses due to frequency and harmonics in DC. The frequency of DC is zero and voltage are constant in DC so DC is more efficient than AC

There are many reasons

• DC transmission has very losses as compared to AC since current flows steadily. There are no reactive losses or dielectric losses. Thus, more power can transmitted over long distances with lesser voltage drop.
• The entire conductor is used for transmission. The conductor cross section is smaller compared to similar AC system. Insulation required is lesser and gives more right of way than AC lines. Hence less land required to be acquired.
• It doesn't produce much noise and interfere with communication lines.
• Chances for power surges are less.
• Only two conductors needed for transmission

But I think dc power transmission is more costly due the cost of equipments used

this case is for High voltage DC, not in general, 

there are many reasons that can be taken into consideration when we compare between the two technologies, as an example of the advantages of using DC: 

The used cables in DC need less cross section area because there is no skin effect in DCalso we need 2 cables instead of 3 cables per circuit, which means lower cost. 

On the other hand the initial cost of implementing such technology is very high with compare to the existing AC grid